NATURE OF THE ACTION CURRENT IN NITELLA : II. SPECIAL CASES

The action curve involves four movements each of which shows considerable variation. These variations can be accounted for on the assumption that the action curve is due to the movement of potassium ions accompanied by an increase in permeability.     

NATURE OF THE ACTION CURRENT IN NITELLA : I. GENERAL CONSIDERATIONS

The outstanding features of the action curve in Nitella are explained as due to the movement of potassium ions accompanied by increase of permeability. This may be useful as a working hypothesis since it accounts not only for the normal behavior but also for many striking deviations which will be treated in subsequent papers. The views here set forth are in harmony with the local circuit theory of stimulation.     

NATURE OF THE ACTION CURRENT IN NITELLA : V. PARTIAL RESPONSE AND THE ALL-OR-NONE LAW

When a stimulus arrives before recovery is complete there may be no response or only a partial response. A typical response appears to involve an immediate loss of potential at the inner protoplasmic surface but not at the outer surface. As long as recovery is incomplete only a part of the total potential is located at the inner protoplasmic surface and the loss of this part of the total potential can cause only a partial response; i.e., one of smaller magnitude than the normal. Even after the action curve has returned to the base line recovery may be incomplete and the response only a partial one. The return of the action curve to the base line means a recovery of total potential but if part of this is located at the outer protoplasmic surface and if this part is not lost when stimulation occurs the response can be only a partial one. During recovery there is a shift of potential from the outer to the inner protoplasmic surface. Not until this shift is completed can recovery be called complete. The response to stimulation then becomes normal because the loss of potential reaches the normal amount. In many cases the partial responses appear to conform to the all-or-none law. In other cases this is doubtful.     

NATURE OF THE ACTION CURRENT IN NITELLA : III. SOME ADDITIONAL FEATURES

Several forms of the action curve are described which might be accounted for on the ground that the outer protoplasmic surface shows no rapid electrical change. This may be due to the fact that the longitudinal flow of the outgoing current of action is in the protoplasm instead of in the cellulose wall. Hence the action curve has a short period with a single peak which does not reach zero. On this basis we can estimate the P.D. across the inner and outer protoplasmic surfaces separately. These P.D.''s can vary independently. In many cases there are successive action currents with incomplete recovery (with an increase or decrease or no change of magnitude). Some of the records resemble those obtained with nerve (including bursts of action currents and after-positivity).     

NATURE OF THE ACTION CURRENT IN NITELLA : IV. PRODUCTION OF QUICK ACTION CURRENTS BY EXPOSURE TO NACl

Treatment of Nitella with NaCl greatly reduces the time required for the action current and produces an action curve with one peak instead of the customary two. The time may be reduced to 0.6 second in place of the usual 15 to 30 seconds. This might be expected if the treatment increased the conductivity of the aqueous part of the protoplasm. The experiments favor this idea although they do not prove its correctness. This effect is prevented by calcium, possibly because calcium inhibits penetration of salts. That penetration is an important factor is indicated by the fact that salts which might be expected to penetrate rapidly have the most effect. Thus NaSCN is more effective than NaCl but Na₂SO₄ has little or no effect. The action of NH₄Cl and LiCl is similar to that of NaCl.     

CHEMICAL RESTORATION IN NITELLA : II. RESTORATIVE ACTION OF BLOOD

Cells of Nitella exposed to distilled water lose their ability to produce action currents and to distinguish electrically between sodium and potassium. This ability was quickly restored by exposure to blood plasma deprived of calcium. Human blood and that of the cat, calf, and sheep gave essentially the same results. The active agents appear to be organic substances.     

THE TOXIC ACTION OF COPPER ON NITELLA

1. Using the loss of turgidity of the cells as a criterion it is found that the toxicity curve of copper chloride with Nitella is sigmoid. An empirical equation can be constructed which will approximately fit the curve. 2. When the concentration of the copper chloride is varied the toxic effect varies as a constant, fractional, power of the concentration. This relation holds when the concentration is plotted against either (1) the time necessary to reach a given point on the ordinate of the survivor curve, (2) the maximum speed of toxic action as shown by the tangent to the survivor curve or (3) the first derivative of the equation which fits the survivor curve. 3. When the temperature is varied and the logarithm of the reciprocal of the time necessary to reach a given point on the survivor curves is plotted against the reciprocal of the absolute temperature the resulting figure consists of several intersecting curves. A hypothetical system is described which will give straight lines under normal conditions and curves when acted upon by a toxic agent.     

PATTERNS OF VARIATION IN THE STELLARIA LONGIPES COMPLEX: EFFECTS OF POLYPLOIDY AND NATURAL SELECTION

Patterns of morphological variation were studied in herbarium specimens of Stellaria longipes, an herbaceous perennial and subsequently in a growth chamber experiment using three cytotypes (4x, 6x, 8x) of S. longipes and diploids of its proposed progenitor S. longifolia. Despite extensive phenotypic plasticity in many traits, patterns of variation resulting from ecotypic differentiation within S. longipes could be detected in the field. A distinct form of S. longipes, which is restricted primarily to arctic and alpine tundra locations, shows genetic differentiation for the following traits: few flowers per ramet, a low proportion of flowering ramets, and ovate leaves. The three cytotypes of S. longipes could be distinguished by their mean genotypic value for leaf length and number of flowers per ramet. The extent of phenotypic plasticity in these traits makes it unlikely that the cytotypes could be distinguished in the field. The direction and extent of morphological divergence between S. longifolia and S. longipes suggest an alloploid origin for S. longipes. Variational trends (among-habitat types and cytotypes) in trait means are similar to those reported previously for the pattern of plasticity. This supports the argument that similar forces guide evolution of the mean and pattern of plasticity of a trait.     

SEXUAL SELECTION AND THE EVOLUTION OF COMPLEX COLOR PATTERNS IN DRAGON LIZARDS

Many species have elaborate and complex coloration and patterning, which often differ between the sexes. Sexual selection may increase the size or intensity of color patches (elaboration) in one sex or drive the evolution of novel signal elements (innovation). The latter potentially increases color pattern complexity. Color pattern complexity may also be influenced by ecological factors related to predation and environment; however, very few studies have investigated the effects of both sexual and natural selection on color pattern complexity across species. We used a phylogenetic comparative approach to examine these effects in 85 species and subspecies of Australian dragon lizards (family Agamidae). We quantified color pattern complexity by adapting the Shannon–Wiener diversity index. There were clear sex differences in color pattern complexity, which were positively correlated with both sexual dichromatism and sexual size dimorphism, consistent with the idea that sexual selection plays a significant role in the evolution of color pattern complexity. By contrast, we found little evidence of a link between environmental factors and color pattern complexity on body regions exposed to predators. Our results suggest that sexual selection rather than natural selection has led to increased color pattern complexity in males.     

NOTE ON THE NATURE OF THE CURRENT OF INJURY IN TISSUES

Leading off from two places on the same cell (of Nitella) with 0.001 M KCl we observe that a cut produces only a temporary negative current of injury. If we lead off with 0.001 M KCl from any cell to a neighboring cell we find that when sap comes out from the cut cell and reaches the neighboring intact cell a lasting negative "current of injury" is produced. This depends on the fact that the intact cell is in contact with sap at one point and with 0.001 M KCl at the other (this applies also to tissues composed of small cells). If we employ 0.1 M KCl in place of 0.001 M the current of injury with a single cell is positive (and is more lasting when a neighboring cell is present). Divergent results obtained with tissues and single cells may be due in part to these factors.     

THE EFFECTS OF CURRENT FLOW ON BIOELECTRIC POTENTIAL : III. NITELLA

String galvanometer records show the effect of current flow upon the bioelectric potential of Nitella cells. Three classes of effects are distinguished. 1. Counter E.M.F''S, due either to static or polarization capacity, probably the latter. These account for the high effective resistance of the cells. They record as symmetrical charge and discharge curves, which are similar for currents passing inward or outward across the protoplasm, and increase in magnitude with increasing current density. The normal positive bioelectric potential may be increased by inward currents some 100 or 200 mv., or to a total of 300 to 400 mv. The regular decrease with outward current flow is much less (40 to 50 mv.) since larger outward currents produce the next characteristic effect. 2. Stimulation. This occurs with outward currents of a density which varies somewhat from cell to cell, but is often between 1 and 2 µa/cm.² of cell surface. At this threshold a regular counter E.M.F. starts to develop but passes over with an inflection into a rapid decrease or even disappearance of positive P.D., in a sigmoid curve with a cusp near its apex. If the current is stopped early in the curve regular depolarization occurs, but if continued a little longer beyond the first inflection, stimulation goes on to completion even though the current is then stopped. This is the "action current" or negative variation which is self propagated down the cell. During the most profound depression of P.D. in stimulation, current flow produces little or no counter E.M.F., the resistance of the cell being purely ohmic and very low. Then as the P.D. begins to recover, after a second or two, counter E.M.F. also reappears, both becoming nearly normal in 10 or 15 seconds. The threshold for further stimulation remains enhanced for some time, successively larger current densities being needed to stimulate after each action current. The recovery process is also powerful enough to occur even though the original stimulating outward current continues to flow during the entire negative variation; recovery is slightly slower in this case however. Stimulation may be produced at the break of large inward currents, doubtless by discharge of the enhanced positive P.D. (polarization). 3. Restorative Effects.—The flow of inward current during a negative variation somewhat speeds up recovery. This effect is still more strikingly shown in cells exposed to KCl solutions, which may be regarded as causing "permanent stimulation" by inhibiting recovery from a negative variation. Small currents in either direction now produce no counter E.M.F., so that the effective resistance of the cells is very low. With inward currents at a threshold density of some 10 to 20 µa/cm.², however, there is a counter E.M.F. produced, which builds up in a sigmoid curve to some 100 to 200 mv. positive P.D. This usually shows a marked cusp and then fluctuates irregularly during current flow, falling off abruptly when the current is stopped. Further increases of current density produce this P.D. more rapidly, while decreased densities again cease to be effective below a certain threshold. The effects in Nitella are compared with those in Valonia and Halicystis, which display many of the same phenomena under proper conditions. It is suggested that the regular counter E.M.F.''S (polarizations) are due to the presence of an intact surface film or other structure offering differential hindrance to ionic passage. Small currents do not affect this structure, but it is possibly altered or destroyed by large outward currents, restored by large inward currents. Mechanisms which might accomplish the destruction and restoration are discussed. These include changes of acidity by differential migration of H ion (membrane "electrolysis"); movement of inorganic ions such as potassium; movement of organic ions, (such as Osterhout''s substance R), or the radicals (such as fatty acid) of the surface film itself. Although no decision can be yet made between these, much evidence indicates that inward currents increase acidity in some critical part of the protoplasm, while outward ones decrease acidity.     

WATER RELATIONS IN THE CELL : I. THE CHLOROPLASTS OF NITELLA AND OF SPIROGYRA

Chloroplasts may contract under natural conditions and give up water to the rest of the cell, thus indicating changes in metabolism or constitution. Such contractions may be produced experimentally. In Nitella the chloroplasts are ellipsoid bodies which, under natural conditions, may contract to spheres with a loss of volume. This may be brought about by lead acetate, ferric chloride, and digitonin: the contraction may occur while the cell is alive. The contraction in lead acetate is reversible (in lead nitrate little or no contraction occurs). In Spirogyra the chloroplast is a long, spirally coiled ribbon which may contract under natural conditions to a short nearly straight rod with a loss of volume. This can be brought about by inorganic salts and in other ways while the cell is still alive.     

ACTION CURVES WITH SINGLE PEAKS IN NITELLA IN RELATION TO THE MOVEMENT OF POTASSIUM

In Nitella the action curve has two peaks, apparently because both protoplasmic surfaces (inner and outer) are sensitive to K⁺. Leaching in distilled water makes the outer surface insensitive to K⁺. We may therefore expect the action curve to have only one peak. This expectation is realized. The action curve thus obtained resembles that of Chara which has an outer protoplasmic surface that is normally insensitive to K⁺. The facts indicate that the movement of K⁺ plays an important part in determining the shape of the action curve.     

CYTOTAXONOMY OF THE CHARACEAE: KARYOTYPE ANALYSIS OF NITELLA OPACA AND NITELLA FLEXILIS

The haploid chromosome complement of Nitella flexilis (n = 12) is composed of two quite different basic karyotypes. One of these is symmetrical and appears to be identical to the karyotype of N. opaca (n = 6), and the other is quite asymmetrical and seems to be identical to that of an apparently undescribed dioecious Nitella from Kansas (n = 6). This may indicate that the monoecious N. flexilis has arisen through hybridization between two dioecious species. Although heteromorphic sex chromosomes were not observed in either species, it appears that female and male potentialities are confined to separate basic chromosome complements and mechanisms determining the monoecious and dioecious states are in close relation to ploidy.     

STEWARTIOTHECA GEN. N. AND THE NATURE AND ORIGIN OF COMPLEX PERMINERALIZED MEDULLOSAN POLLEN ORGANS

Stewartiotheca gen. n. is a bell-shaped, unisynangiate pollen organ with eccentric radial symmetry and a single series of about 80 pollen sacs. Infoldings that vary in depth occur circumferentially and extend from the periphery to a point off center. This position also marks the location of a sclerenchyma column (proximally) and a sclerenchyma-lined, conical hollow (distally) that opens onto the distal face of the organ. Plates of ground parenchyma extend inward from the outer covering of the organ at locations of infoldings, while similar plates with sclerenchyma strands occur between these locations. Pre-pollen of Monoletes type was released through distal longitudinal slitlike openings of the pollen sac faces toward the sclerenchymatous ground tissue plates. Vascular bundles entering the organ undergo repeated dichotomies, and lead to numerous bundles both in the cover (one per sac for those sacs that abut directly on cover tissue) and internally (one per pair of pollen sacs that lie opposite one another across the location of an infolding). The most complex permineralized medullosan pollen organs Sullitheca, Stewartiotheca, and Dolerotheca are considered to have evolved from a similar type of cup-shaped organ with a single ring of pollen sacs, broadly open distally, and with a central hollow. Circumferential infoldings of one organ of this type were involved in the origin of both Stewartiotheca and Sullitheca, while four similar organs, each showing infoldings non-circumferentially, fused to produce the Dolerotheca type organ (exemplified by D. formosa), a compound synangium.     

COMPLEX AND MODIFIABLE BEHAVIOR PATTERNS IN CALLIACTIS AND STOMPHIA

Three behavior patterns in sea anemones are analyzed in relationto the receptors, elfectors, and co-ordinating s)stems requiredfor their perfomance: Calliactis responding to molluscan "shell-factor." Stomphia swimming to starfish and to electrical stimulation. Stomphia transferring to shells of its commensal Modiolus modiolus. These complex purposive acts are triggered and controlled bythe excitation of chemoreceptors. Co-ordinating nervous componentshave not been identified. These specific responses offer possibilitiesfor tests on learning for which unequivocal evidence does notexist in coelenterates.     

THE DEATH WAVE IN NITELLA : I. APPLICATIONS OF LIKE SOLUTIONS.

Experiments on cutting confirm the prediction that the current of injury will be positive when the cell is in contact with concentrated solutions and negative with dilute solutions. They support the idea that the protoplasm is made up of layers differing considerably in their properties, each having a death curve of simple and regular form, the more rapid alteration of the outer layer making the protoplasm more positive and the more rapid alteration of the inner making it more negative. From the point where the cell is cut a wave of some sort, which we may for convenience call a death wave, passes along the cell, setting up at each point it touches a death process which has the greater speed and intensity the nearer it is to the cut.     

Studies on Plant-Growth Hormones: VI. THE NATURE OF INHIBITOR-{beta} IN POTATO

1. The chemical nature of a plant growth inhibitor in potato tuberpeel, the ‘Inhibitor-ß’ which commonlyoccurs on chromatograms of many plant extracts, has been examined.
2. The inhibition, as indicated by bioassays using wheat coleoptilesections, could not be associated with any particular compound,but was partly or entirely due to a complex mixture of aliphaticacids.
3. . Azelaic acid and the coumarin, acopoletin, were isolatedtogetherwith a new substance, Acid A; degradative evidenceis not sufficientto enable a complete structure to be proposedfor this acid,but it appears to be an unsaturated polyhydroxyfatty acid.
4. The growth of coleoptile sections in solutionsof ßat several concentrations was examined over thefirst 7 hoursof growth. Inhibition did not occur until 4 hours;visible damageto the cells of the tissues appeared after thisperiod. Whenß was examined in a mixture with 3-indolylaceticacid,inhibition was evident after 1 hour. These results areinterpretedand the chemical system in which ß mayoperate ingrowth is briefly considered.

     

CINEMATIC OBSERVATIONS ON THE GROWTH AND DIVISION OF CHLOROPLASTS IN NITELLA

As it elongates from about 0.2 to 80 mm, the Nitella internodal cell shows an increase in plastid number from a few thousand to about 4 million. The increase takes place by plastid division. A continuous motion picture record followed a population of 8 plastids in an elongating cell until their progeny numbered 18, a span longer than 1 fission cycle for some of the plastids. One complete fission-fission cycle was about 22 hr. The highly directed nature of chloroplast expansion (elongation) is lost when cell wall strain (expansion) is mechanically inhibited by pressing the cell between glass plates. The plastids then expand about equally in all directions in the plane of the cell surface. When a new direction of maximum strain is introduced by the mechanical induction of a lateral in the cell, the plastids elongate in this new direction. The direction of the protoplasmic stream does not show this striking response to strain but tends to follow the lines of the chloroplast chains, not the long axis of individual plastids.