THE RATE OF GROWTH OF THE DAIRY COW : V. EXTRAUTERINE GROWTH IN LINEAR DIMENSIONS.

Barring fluctuations due to the cyclic phenomena, the extrauterine course of growth in linear dimensions and in weight of the dairy cow follows an exponential law having the same form as the law representing the course of monomolecular change in chemistry. This suggests the interpretation that the general course of growth is limited by a monomolecular chemical process, and that the cyclic phenomena are due to subsidiary processes in the fundamentally exponential course of growth. The fact that growth follows or tends to follow an exponential course may be stated more simply as follows: if the unit of time is taken sufficiently large so that fluctuations due to the cyclic phenomena are balanced or eliminated, then the amount of growth made during the given unit of time at any age tends to be a constant percentage of the growth made during the preceding unit of time. Thus, the growth in height at withers made during any year is about 34 per cent of the growth made during the preceding year. Similarly the growth in weight made during any year is about 56 per cent of the growth in weight made during the preceding year. This is in accordance with expectations if it is assumed that each animal begins life with a definite endowment of limiting substance necessary for the process of growth, and that this endowment is used up at a constant rate (or percentage) of itself.     

EFFECT OF UNDERNUTRITION ON CELL FORMATION IN THE RAT BRAIN

Abstract— Rats were undernourished by approximately halving the normal food given from the 6th day of gestation throughout lactation. Growth of the foetuses was nearly normal, in marked contrast to the severe retardation caused by undernutrition during the suckling period. In comparison with controls the size and the DNA content of the brain were permanently reduced by undernutrition during the suckling period: this effect was relatively small, approx. 15 per cent decrease at 21 and 35 days. The rate of ¹⁴C incorporation into brain DNA at 30 min after administration of [2-¹⁴C] thymidine was taken as an index of mitotic activity; compared with controls there was severe reduction in mitotic activity (maximal decrease by about 80 per cent at 6 days in the cerebrum and by 70 per cent at 10 days in the cerebellum). The rate of acquisition of cells was calculated from the slopes of the logistic curves fitted to the estimated DNA contents. In normal animals the maximal slope was attained at 2·7 days and at 12·8 days after birth in cerebrum and cerebellum respectively; the daily acquisition of cells at these times was 4·8 × 10⁶ and 18 × 10⁶ cells respectively. The fractional increase in cell number at the maximum was 5·4 percent per day in the cerebrum and 15·2 per cent per day in the cerebellum. The rate of acquisition of cells relative to the rate of mitotic activity was higher in the brains of undernourished animals than in controls. One of the compensatory mechanisms for the severe depression of mitotic activity in the brain of undernourished animals Seems to involve a reduction in the normal rate of cell loss.     

COMPARATIVE STUDIES ON RESPIRATION : III. THE EFFECT OF ETHER ON THE RESPIRATION AND GROWTH OF BACILLUS SUBTILIS.

1. In all the concentrations of ether studied (from 0.037 to 7.3 per cent) there is an increase in the rate of respiration of Bacillus subtilis followed by a decrease. 2. In 7.3 per cent ether in tap water there is an extraordinary increase in the output of CO₂ (amounting to 50 times the normal). This does not occur when 0.85 per cent NaCl is added, which indicates antagonism between ether and NaCl. 3. Ether is toxic in low concentrations (0.037 to 1.1 per cent) and high concentrations (3.65 to 7.3 per cent) but in intermediate concentrations (1.1 to 3.65 per cent) stimulates growth.     

TIME RELATIONS OF GROWTH : I. GENETIC GROWTH CONSTANTS OF ANIMALS.

This paper is the first in a series proposing to evaluate growth constants from the viewpoint of and by the methods of the physical chemist. After discussing the physicochemical conception of growth, tentative numerical values of two growth constants are given for several species of animals including man, and the methods of computation are discussed in detail.     

THE EFFECT OF AGE, GROWTH RETARDATION AND ASPHYXIA ON ASCORBIC ACID CONCENTRATIONS IN DEVELOPING BRAIN

Abstract— The concentration of ascorbic acid in whole rat brain during the first week of postnatal life was up to 100 per cent higher than in adult animals. A progressive fall in concentration occurred between 4 and 30 days of age. Corresponding changes did not occur in liver and adrenal gland, two other organs rich in ascorbic acid. Rats subjected to growth retardation during the fetal and suckling periods had, at 25 days of age, levels of ascorbic acid in the cerebellum and brainstem significantly higher than those of control animals. A period of prolonged asphyxia in 5-day-old rats resulted in a significant 4 per cent reduction in whole brain ascorbic acid concentration.     

COMPARATIVE STUDIES ON RESPIRATION : V. THE EFFECT OF ETHER ON THE PRODUCTION OF CARBON DIOXIDE BY ANIMALS.

1. The experiments on frog tadpoles show that with 0.15, 0.37, and 0.55 per cent ether solutions there is a decrease in CO₂ output. The effect is reversible. With these concentrations the breathing movements and body movements remained normal during the experiment. In 3.65 and 7.3 per cent ether there is a decrease of respiration followed by an increase which in turn is followed by a decrease. The increase may reach about three times the normal rate. The increase in the CO₂ output is accompanied by the peeling of the skin. The effect is irreversible. 2. Experiments on an aquatic insect, Dineutes assimilis Aube, show that in 7.3 per cent ether there is a decrease followed by an increase which in turn is followed by a decrease. There is no apparent disintegration of structures in the organism accompanying the increase. The effect is irreversible. 3. The experiments on frog eggs with 7.3 per cent ether show a result similar to that found in aquatic insects. 4. Experiments on Fundulus embryos show that with 0.73 per cent ether there is a reversible decrease in the rate of CO₂ production. In 3.65 per cent ether there is a temporary decrease followed by an increase, after which the rate begins to fall off. In 7.3 per cent ether there is an immediate increase amounting to 307 per cent which is followed by a decrease. The increase in the 3.65 and 7.3 per cent ether is accompanied by irreversible changes leading to death. The decrease found in 0.73 per cent ether is not sufficient to cause narcosis, as is shown by experiments on which the same decrease is produced by lowering the temperature. 5. These experiments show that narcosis is not due to asphyxia. The action of anesthetics is due to some other cause than the effect on respiration. There is a difference between the animals studied and the plants described in this series of articles, since in animals the increase in the CO₂ output is accompanied by irreversible changes leading to death, while this is not necessarily the case in plants. The reversible (narcotic) action of ether on the animals studied was accompanied by a decrease in the carbon dioxide output; in plants this is not ordinarily the case. These facts are of considerable interest, but their interpretation must be left to future investigation.     

THE OXYGEN CONSUMPTION ASSOCIATED WITH GROWTH IN ESCHERICHIA COLI AND THE EFFECT OF SULFATHIAZOLE AND OF n-PROPYL CARBAMATE ON IT

1. The rate of oxygen consumption by E. coli has been observed both in the presence and absence of ammonia which substance is used by this organism in these experiments as the sole source of nitrogen for growth. 2. After the ammonia has been completely taken up in a culture of E. coli, the rate of oxygen consumption by the culture is observed to fall rapidly. It becomes relatively constant again at a rate approximately 45 per cent of that existing immediately prior to the exhaustion of the nitrogen source. It appears that the fixation of ammonia, that is, growth, requires approximately 55 per cent of the oxygen consumed by the growing cell. 3. Inhibition of the oxygen consumption which is associated with ammonia fixation, by both sulfathiazole (ST) and n-propyl carbamate (PC) closely parallels the inhibition of growth by these compounds (as measured by viable cell counts, etc.). 4. The concentrations of ST and PC which inhibit growth exert little or no inhibitory effect on the rate of oxygen consumption by cells after the rate has fallen to the resting value. 5. It is pointed out that the above observations would be adequately accounted for if growth depended on a discrete fraction of the total oxygen consumption of the growing cell. 6. It is noted that PC, but not ST, has a significant accelerating effect on the oxygen consumption of the resting cell; and that for a given inhibition of growth, PC produces less inhibition of the total oxygen consumption of the cells, than does ST. The latter of these two observations would follow from the former if the resting oxygen consumption were a discrete entity.     

THE INFLUENCE OF ALCOHOL UPON THE GROWTH OF SEEDLINGS

In this paper it is shown that if the dry seeds of the cantaloupe (Cucumis melo) are soaked for 3 hours in solutions of ethyl alcohol of concentration ranging from 2 to 16 per cent by volume, and then germinated and grown in distilled water in the dark, the total growth attained is greater by amounts ranging from 9 to 35 per cent than is that made by seeds treated in every way identically except that they are initially soaked in distilled water instead of alcohol. It is shown that this result is not due simply to differences in osmotic pressure in the different alcohol solutions. It is probably due to a simple selective action of the alcohol which eliminates the constitutionally weak and defective seeds.     

A morphometric analysis of the spinal motor pool in relation to its target muscle during growth in the trout, Salmo gairdneri Richardson

The relation between number and size of spinal motoneurons and the dimension of myotomal muscle has been investigated in trout at different stages of embryonic, larval and postlarval development (body length 1–15 cm). Three spinal segments have been analysed (cervical, trunk and caudal) and the following parameters were determined by means of a Micromeasurements Image Analyzer: (a) mean cross-sectional myotomal area; (b) mean soma size of principal (or dorsomedian, DM) and secondary (or ventrolateral, VL) motoneurons; (c) DM and VL motoneuron density per segment. Myotomal muscle and motor pool growth was evaluated by percent increments of a, b and c parameters at each stage. Their relationships were denned by equations of computed regression lines.
The analysis provided evidence that: (1) a continuous exponential growth of mean myotomal area takes place in the three segments, with the same trend and with the lowest values in the caudal segment; (2) DM and VL motoneuron size and density per segment also increase during development, with the least value in the caudal segment, VL parameters being of lesser value than DM; (3) motoneuron pool and its target myotomal muscle parameters bear a linear relationship as defined by equations of computer regression lines; (4) motoneuron number percent increment at early eleutherembryonic stage precedes myotomal area increment which takes place during late eleutherembryonic stage.
It is apparent that spinal motor pool and target myotomal muscle grow at the same rate in the trout during the considered stages. The discussion links this fact with the hypothesis of a neuronal influence on muscle fibre type differentiation.     

THE EFFECTS OF SULFATHIAZOLE AND OF PROPYL CARBAMATE ON THE RATE OF OXYGEN CONSUMPTION AND GROWTH IN ESCHERICHIA COLI

1. The rates of growth and of oxygen consumption by cells of E. coli have been measured under identical conditions, and the effects of sulfathiazole (ST) and of n-propyl carbamate (PC) on these two processes have been compared. 2. The rate of growth was measured by (a) the increase in the viable cell count, (b) the increase in the optical density of the culture, (c) the increase in the rate of oxygen consumption, and (d) the decrease in the ammonia of the medium. The results as indicated by these several measures were identical under the conditions of these experiments. 3. Concentrations of ST or of PC which are just sufficient to stop growth completely, lower the rate of oxygen consumption per unit of bacterial protoplasm to a value approximately 50 per cent of that seen in the absence of the inhibitor. 4. It is shown that the rate of oxygen consumption in cells from old cultures is less affected by ST than is the rate of oxygen consumption by cells from young cultures. It is probable that the rate of oxygen consumption by "old" cells is lower than that of "young" cells. 5. The effects of ST and PC on both the rate of oxygen consumption and the rate of growth are very similar, indicating in a general way, that the mechanism of the actions of these two inhibitors is similar. Furthermore, since both of them produce appreciable inhibition of the rate of oxygen consumption while they are inhibiting growth, the possibility that the effect on oxygen consumption is the immediate cause of the effect on growth must be entertained.     

THE INFLUENCE OF ENVIRONMENTAL TEMPERATURE ON THE UTILIZATION OF FOOD ENERGY IN BABY CHICKS

1. An optimum of environmental temperature is to be expected for the utilization of food energy in warm blooded animals if their food intake is determined by their appetite. 2. Baby chicks were kept in groups of five chicks in a climatic cabinet at environmental temperatures of 21°, 27°, 32°, 38°, and 40°C. during the period of 6 to 15 days of age. The intake of qualitatively complete food was determined by their appetite. Food intake, excretion, and respiratory exchange were measured. Control chicks from the same hatch as the experimental groups were raised in a brooder and were given the same food as the experimental chicks. The basal metabolism of each experimental group was determined from 24 to 36 hours without food at the age of 16 days. 3. The daily rate of growth increased with decreasing environmental temperature from 2.74 gm. at 40°C. to 4.88 gm. at 21°C. This was 4.2 to 6.5 per cent of their body weight. 4. The amount of food consumed increased in proportion to the decrease in temperature. 5. The availability of the food, used for birds instead of the digestibility and defined as See PDF for Structure showed an optimum at 38°C. 6. The CO₂ production increased from 2.95 liters CO₂ per day per chick at 40°C. to 6.25 liters at 21°C. Per unit of the 3/4 power of the body weight, 23.0 liters CO₂ per kilo^3/4 was produced at 40°C. and 43.4 liters per kilo^3/4 at 21°C. The CO₂ production per unit of 3/4 power of the weight increased at an average rate of approximately 1 per cent per day increase in age. The R.Q. was, on the average, 1.04 during the day and 0.92 during the night. 7. The net energy is calculated on the basis of C and N balances. A maximum of 11.8 Cal. net energy per chick per day was found at 32°C. At 21°C. only 6.9 Cal. net per day per chick was produced and at 40°C. an average of 6.7 Cal. 8. The composition of the gained body substance changed according to the environmental temperature. The protein stored per gram increase in body weight varied from 0.217 to 0.266 gm. protein and seemed unrelated to the temperature. The amount of fat per gram gain in weight dropped from a maximum of 0.153 gm. at 32°C. to 0.012 gm. at 21°C. and an average of 0.107 gm. at 40°C. The energy content per gram of gain in weight had its maximum of 2.95 Cal. per gm. at 38°C. and its minimum of 1.41 Cal. per gm. at 21°C. at which temperature the largest amount of water (0.763 gm. per gm. increase in body weight) was stored. 9. The basal metabolism increased from an average of 60 Cal. per kilo^3/4 at an environmental temperature of 40°C. to 128 Cal. per kilo^3/4 at 21°C. No indication of a critical temperature was found. 10. The partial efficiency, i.e. the increase in net energy per unit of the corresponding increase in food energy, seemed dependent on the environmental temperature, reaching a maximum of 72 per cent of the available energy at 38°C. and decreasing to 57 per cent at 21°C. and to an average of 60 per cent at 40°C. 11. The total efficiency, i.e. the total net energy produced per unit of food energy taken in, was maximum (34 per cent of the available energy) at 32°C., dropped to 16 per cent at 21°C., and to an average of 29 per cent at 40°C.     

ON THE GROWTH AND RESPIRATION OF SULFUR-OXIDIZING BACTERIA

1. It is shown that Sulfomonas thiooxidans oxidizes elementary sulfur completely to sulfuric acid. Sodium thiosulfate is oxidized by this organism completely to sulfate. Sulfomonas thiooxidans differs, in this respect, from various other sulfur-oxidizing bacilli which either produce elementary sulfur, from the thiosulfate, or convert it into sulfates and persulfates. 2. The organism derives its carbon from the CO₂ of the atmosphere, but is incapable of deriving the carbon from carbonates or organic matter. 3. The S:C, or ratio between the amount of sulfur oxidized to sulfate and amount of carbon assimilated chemosynthetically from the CO₂ of the atmosphere, is, with elementary sulfur as a source of energy, 31.8, and with thiosulfate 64.2. The higher ratio in the case of the thiosulfate is due to the smaller amount of energy liberated in the oxidation of sulfur compound than in the elementary form. 4. Of the total energy made available in the oxidation of the sulfur to sulfuric acid, only 6.65 per cent is used by the organism for the reduction of atmospheric CO₂ and assimilation of carbon. 5. Sulfates do not exert any injurious effect upon sulfur oxidation by Sulfomonas thiooxidans. Any effect obtained is due to the cation rather than the sulfate radical. Nitrates exert a distinctly injurious action both on the growth and respiration of the organism. 6. There is a definite correlation between the amount of sulfur present and velocity of oxidation, very similar to that found in the growth of yeasts and nitrifying bacteria. Oxidation reaches a maximum with about 25 gm. of sulfur added to 100 cc. of medium. However, larger amounts of sulfur have no injurious effect. 7. Dextrose does not exert any appreciable injurious effect in concentrations less than 5 per cent. The injurious effect of peptone sets in at 0.1 per cent concentration and brings sulfur oxidation almost to a standstill in 1 per cent concentration. Dextrose does not exert any appreciable influence upon sulfur oxidation and carbon assimilation from the carbon dioxide of the atmosphere. 8. Sulfomonas thiooxidans can withstand large concentrations of sulfuric acid. The oxidation of sulfur is affected only to a small extent even by 0.25 molar initial concentration of the acid. In 0.5 molar solutions, the injurious effect becomes marked. The organism may produce as much as 1.5 molar acid, without being destroyed. 9. Growth is at an optimum at a hydrogen ion concentration equivalent to pH 2.0 to 5.5, dropping down rapidly on the alkaline side, but not to such an extent on the acid, particularly when a pure culture is employed. 10. Respiration of the sulfur-oxidizing bacteria can be studied by using the filtrate of a vigorously growing culture, to which a definite amount of sulfur is added, and incubating for 12 to 24 hours.     

Areal growth in the human fetal parietal bone

The areal growth of the human fetal parietal bone is described using 51 dissected right parietal bones of Japanese fetuses ranging from the fifth month to term. Their shadows were taken on printing paper and analyzed by a sonic digitizer. The absolute growth of the projected area of the fetal parietal bones progresses at a fairly constant rate during the latter half of the fetal period, despite some deceleration in the 9–10th month. By allometric analysis, the allometric coefficients against crown-rump length are 2.201 for males, 2.202 for females, and 2.204 for both sexes, respectively. The allometry is essentially monophasic, showing no inflection point, indicating a change in the slope of the regression line; that is, the growth of the human fetal parietal bone is continuous with a constant specific growth rate. These results are discussed in connection with growth in thickness of the bone.     

Gonadotrophic effect of juvenile hormone in Blattella germanica: A rapid,simple quantitative bioassay

The German cockroach is convenient for assaying gonadotrophic effects of juvenile hormone (JH) analogues. The animals can be raised synchronously from egg to adult state in 2·5 months at 30°C, a shorter time than for any of the common laboratory-reared cockroaches. Last instar larvae can be stored for up to a month at 16 to 18°C and brought to 30°C to metamorphose when needed, providing a convenient source of bioassay animals. The assay takes 2 days to complete, less time than previously published gonadotrophic assays. The growth of terminal oöcytes of the ovary responds linearly with log dose between 0·08 and 0·8 μg of Cecropia JH-I. The analogues JH-I ethyl ester and methyl-juvenate have dose response curves parallel to that for JH-I, enabling potencies, r, relative to JH-I to be calculated (r = 1·01, 0·35 respectively). There is no significant effect of the volume of hormone vehicle between 2 and 8 μl. A 30 per cent increase in the precision of the bioassay is obtained by correcting the oöcyte growth for the fed-weight of the animal. The bioassay is adequate to resolve a twofold difference in potency using as few as 36 animals per analogue.     

Radioprotective and antineoplastic activity of polyene antibiotics combined with dimethyl sulfoxide]

Radioprotective and antineoplastic activity of polyene, its derivatives and combinations with dimethyl sulfoxide (DMSO) was studied. The most potent radioprotective effect was demonstrated by methylated levorin, original levorin and by its isomer--isolevorin. Survival rate of the animals on 12th day after X-ray exposure was 100, 60, 60 per cent, at the control group 33.6, 20 and 0 per cent consequently. Levorin and alkyl derivatives of amphotericin B--methamphocin and buthamphocin inhibited growth of ascites and solid tumors to 46.3-79.0 per cent when compared to control group. Polyen antibiotics combined with DMSO also demonstrated antineoplastic activity at the animals treated with carcinogenic agent--diethyl nitrosoamine (DENA). 5-month survival of the animals was 76 per cent at nystatin and levorin group and 35.7 per cent at the control group (animals treated with DENA only).     

STUDIES ON THE GROWTH HORMONE OF PLANTS : VI. THE DISTRIBUTION OF THE GROWTH SUBSTANCE IN PLANT TISSUES

1. It is shown that when plant tissues are ground with water the growth substance contained therein is inactivated by the oxidizing enzymes. 2. A simple method of extraction is described which enables the quantitative determination of growth substance in such tissues. 3. The amount and distribution of growth substance in the Avena coleoptile is determined by this method, and it is shown that while the substance does not diffuse out from the lower parts of the coleoptile, it is nevertheless present in considerable amounts, the concentration decreasing steadily with the distance from the tip. 4. Growth substance is also present in considerable amounts in Avena roots, and here also its concentration decreases steadily with distance from the tip. 5. The amount of growth substance diffusing out of root tips into dextrose agar, even during long periods of time, is not greater than the amount obtainable by direct extraction. Actual production in the root tip therefore either does not take place at all, or else takes place under quite different conditions from the production in the tip of the coleoptile.     

CIRCADIAN GROWTH RHYTHM IN JUVENILE SPOROPHYTES OF LAMINARIALES (PHAEOPHYTA)1

A circadian rhythm in growth was detected by computer-aided image analysis in 3–4-cm-long, juvenile sporophytes of the kelp species Pterygophora California Rupr. and in seven Laminaria spp. In P. californica, the free-running rhythm occurred in continuous white fluorescent light, had a period of 26 h at 10°or 15°C, and persisted for at least 2 weeks in white or blue light. The rhythm became insignificant in continuous green or red light after 3 cycles. Synchronization by white light-dark regimes, e.g. by 16 h light per day, resulted in an entrained period of 24 h and in a shift of the circadian growth minimum into the middle of the light phase. A morning growth peak represented the decreasing portion of the circadian growth curve, and an evening peak the increasing portion. The circadian growth peak was not visible during the dark phase, because growth rate decreased immediately after the onset of darkness. At night, some growth still occurred at 16 or 12 h light per day, whereas growth stopped completely at 8 h light per day, as in continuous darkness. During 11 days of darkness, the thallus area became reduced by 3.5%, but growth rate recovered in subsequent light–dark cycles, and the circadian growth rhythm reappeared in subsequent continuous light.     

大型溞生长、生殖和种群增长的研究

年龄6±6小时的纯系大型溞培养在25±1℃静置换水条件下,饲以斜生栅藻,其平均寿命为68.40±9.82天。龄期(x)和年龄(t,天)之间呈曲线迴归关系:t=-2.245+1.510x+0.035x2(r=0.99,p3.86-0.131t)(式中Y为累计生殖量个数,t为天数)。       

IMPAIRMENT OF GROWTH AND DEVELOPMENT OF THE RAT BRAIN BY HYPEROXIA AT ATMOSPHERIC PRESSURE

Abstract— The continuous exposure of newborn rats to 70–80 per cent oxygen at atmospheric pressure throughout the iirst 9 days of life significantly inhibited the growth of the brain which normally occurs during this period of life. The accumulations of DNA, RNA, total protein, and proteolipid protein which accompany brain growth during this period were all approximately proportionately depressed by the oxygen-enriched atmosphere. RNA/DNA and protein/DNA ratios were unaffected. The increase in brain mass in the first week of life reflects mainly cell proliferation, and since the decreased DNA accumulation occurred with no changes in RNA/DNA and protein/DNA ratios, we conclude that the effect of oxygen was to inhibit cellular division. We estimate that the oxygen exposure caused an approximately 7 per cent deficit in the cell population of the brain. These results indicate that the use of elevated concentrations of oxygen may have serious deleterious effects on the growth and development of the brain.     

RADIATION EFFECTS ON THE GROWTH RATE AND CELL POPULATION KINETICS OF ACTIVELY GROWING AND DORMANT ROOTS OF TRADESCANTIA PALUDOSA

Actively growing and dormant roots of Tradescantia paludosa were exposed to x-rays to compare the radiosensitivity of an actively proliferating tissue with that of one which is not active but is potentially proliferative. The level of effect was ascertained by the degree of change in the rate of root growth 4 days after exposure. Cell population kinetics were measured in control and in irradiated roots to determine whether or not a change was produced either in the number of proliferating cells or in the mitotic cycle duration which was sufficient to explain the altered rate of root growth. Nuclear volumes were also measured to provide an estimate of the relative total target size in actively growing vs. dormant roots. Tritiated thymidine was used to measure the cycle duration and the proportion of cells synthesizing DNA. The results showed that 184 and 305 r respectively were required to reduce the linear root growth rate to 37 per cent of that of the control for actively growing and dormant roots. Mitotic cycle duration, measured 4 days after x-ray exposure, was the same as in the control. The number of proliferating cells, however, was reduced. The rate of cell production in the irradiated roots was reduced to approximately one-half that of the controls. The average nuclear volumes of active and dormant roots were 733 and 491 µ³ respectively; thus the difference in the number of roentgens required to reduce growth to 37 per cent of that of the control can be attributed to the different average nuclear volumes. Therefore, the experiments suggest that part if not most of the differences in sensitivity between an actively dividing and an essentially non-dividing meristematic cell population resides in their different average nuclear volumes. Thus the law of Bergonie and Tribondeau needs to be reinterpreted, since the basic reason for the differences is secondary to whether or not the meristematic cells are proliferating.