A connective tissue tumor in the mussel Mytilus trossulus from a polluted region of Nakhodka Bay, the Sea of Japan

A tumor was found for the first time in a mussel Mytilus trossulus from a heavily polluted area of Nakhodka Bay, Sea of Japan. Tumor cells were found in the connective tissue of different organs and also in gill vessels and hemal sinuses of the visceral mass. They were both attached and diffuse. The tumor was at an advanced stage, replacing the normal connective tissue cells, and formed nodes. The tumor cells were polymorphic, with a high nucleocytoplasmic ratio, and had a prominent nucleolus. The size of their nuclei was three to five times that of the nuclei of agranular hemocytes. The mitotic activity of the tumor cells was more than an order of magnitude higher than in the normal cells: the mean mitotic index was 1.4 +/- 0.5%, ranging from 0.97 to 2.3% in different organs. The mitotic indices in the connective tissue cells of three normal mussels were 0, 0, and 0.12%. A significant proportion (up to 78%) of the mitotic cells were at metaphase. The frequency of abnormal mitoses was 17%. Metaphases with displaced (often multiple) chromosomes constituted 71% of abnormal mitoses; anaphases, 8%; and tri- and tetrapolar mitoses, 11%. The tumor described is similar to diffuse sarcomatoid diseases of mussels from other geographical regions.     

Selective Somatic Elimination of NICOTIANA GLUTINOSA Chromosomes in the F(1) Hybrids of N. SUAVEOLENS and N. GLUTINOSA

The F₁ hybrids of Nicotiana suaveolens (subgenus Petunioides, 2n = 32) and N. glutinosa (subgenus Tabacum, 2n = 24), were examined during their development, from seedlings to mature plants. It was observed that in the hybrids, there was a progressive change of dominant N. glutinosa morphological characteristics towards those of N. suaveolens, in leaf shape, stem, flower color and branching pattern. A study of mitotic chromosomes in the root-tips and in very young anthers of the mature plants indicated a significantly high average frequency of aberrant mitotic anaphases (bridges and fragments, 12% and 11% respectively). As a consequence of this phenomenon, variability in the number and size of chromosomes was observed in the PMC's and in mitotic metaphases (29-24 chromosomes). In order to establish whether the N. glutinosa chromosomes were preferentially lost, a karyological study of the parents and their F₁ hybrids was carried out and it was established that the F₁ hybrids were losing N. glutinosa chromosomes preferentially. A mechanism was suggested for the loss of these chromosomes by means of a chromatid type of breakage-fusion-bridge cycle (b-f-b cycle) and initiation of the b-f-b cycle in the hybrid due to an interaction of the regulatory mechanism of DNA replication in the haploid genomes of the parental species. However, loss of these chromosomes owing to interaction of certain genes from the two parental species cannot be ruled out.     

Neoplasia in the connective tissue of the musselMytilus trossulus from polluted areas of Nakhodka Bay, Sea of Japan

A tumor was found for the first time in a musselMytilus trossulus from aheavily polluted area of Nakhodka Bay, Sea of Japan. Tumor cells were found in the connective tissue of different organs and also in gill vessels and hemal sinuses of the visceral mass. They were both attached and diffuse. The tumor was at an advanced stage, replacing the normal connective tissue cells, and formed nodes. The tumor cells were polymorphic, with a high nucleocytoplasmic ratio, and had a prominent nucleolus. The size of their nuclei was three to five times that of the nuclei of agranular hemocytes. The mitotic activity of the tumor cells was more than an order of magnitude higher than in the normal cells: The mean mitotic index was 1.4±0.5%, ranging from 0.97 to 2.3% in different organs. The mitotic indices in the connective tissue cells of three normal mussels were 0, 0, and 0.12%. A significant proportion (up to 78%) of the mitotic cells were at metaphase. The frequency of abnormal mitoses was 17%. Metaphases with displaced (often multiple) chromosomes constituted 71% of abnormal mitoses; anaphases, 8%; and tri- and tetrapolar mitoses, 11%. The tumor described is similar to diffuse sarcomatoid diseases of mussels from other geographical regions.     

Evaluation of cytological effects of Zn2+ in relation to germination and root growth of Nigella sativa L. and Triticum aestivum L

The effects of different treatments with zinc sulfate (Zn(2+)) on the cytology and growth of Nigella sativa and Triticum aestivum were investigated. Five concentrations of zinc sulfate ranging from 5 to 25mg/l were applied for 6, 12, 18, and 24h. The treatments reduced the germination percentages of N. sativa seeds and T. aestivum grains and inhibited the root growth of both plants. Concentrations higher than 25mg/l of Zn(2+) applied for 24h were toxic for both plants. The non-lethal concentrations of Zn(2+) showed an inhibitory effect on cell division in root tips of both plants and caused a decrease in their mitotic index values. The reduction in MI in root tips of T. aestivum was more evident than that of N. sativa. All treatments changed the frequency of mitotic phases as compared with the control values. The total percentage of abnormalities in N. saliva was more than that in T. aestivum. Zn(2+) treatments produced a number of mitotic abnormalities in dividing cells in root tips of both plants resulting from its action on the spindle apparatus such as C-metaphases, lagging chromosomes and multipolar anaphases and telophases. Also, Zn(2+) induced vacuolated nuclei and irregular prophases. The induction of chromosomal stickiness and chromosomal aberrations such as bridges and breaks indicates its action on the chromosome. These abnormalities (chromosome breaks and chromosomal bridges at ana-telophases) indicate true clastogenic potential of the ions tested.     

Securin is required for chromosomal stability in human cells

Abnormalities of chromosome number are the most common genetic aberrations in cancer. The mechanisms regulating the fidelity of mitotic chromosome transmission in mammalian cells are therefore of great interest. Here we show that human cells without an hSecurin gene lose chromosomes at a high frequency. This loss was linked to abnormal anaphases during which cells underwent repetitive unsuccessful attempts to segregate their chromosomes. The abnormal mitoses were associated with biochemical defects in the activation of separin, the sister-separating protease, rendering it unable to cleave the cohesin subunit Scc1 efficiently. These results illuminate the function of mammalian securin and show that it is essential for the maintenance of euploidy.     

Facial hair whorls (trichoglyphs) and the incidence of motor laterality in the horse

Several species demonstrate obvious motor laterality (sidedness, handedness) in their motor function. Motor laterality in the horse affects locomotion and subsequently equine performance during training and may have inherent safety implications for equitation. Some of the most commonly used identification features in the horse are hair whorls (trichoglyphs), since their specific location and character vary to some degree in every horse. We investigated the relationship between the hair flow of single facial hair whorls and the incidence of lateralised motor bias in 219 horses when under saddle in ridden work. The horses exhibited significant differences in motor preferences with 104 left-lateralised (LL) horses, 95 right-lateralised (RL) horses compared to only 20 well-balanced (WB) horses (chi(2)=36.9, d.f.=2, P<0.01). There was also a significant difference in the frequency distribution of single facial hair whorl patterns in the horses consisting of 114 horses with counter-clockwise (CC) whorls, 82 horses with clockwise (C) whorls and 23 horses, which had radial (R) whorls (chi(2)=38.87, d.f.=2, P<0.01). Overall there was a statistically significant association between motor behaviour and facial hair whorl patterns in the horses (chi(2)=69.4, d.f.=4, P>0.001). The RL horses had significantly more C facial hair whorls and the LL horses had significantly more CC facial hair whorls than would be expected purely by chance alone (P<0.05). The findings may provide trainers with a useful tool when attempting to identify simple, non-invasive and reliable predictors of motor laterality in the horse. Furthermore, given that efficient targeted training of performance horses during ridden work may produce WB equine athletes, the findings could assist trainers when designing individual-specific training programmes for young horses.     

Cytogenetics, Geographical Distribution, Pollen Stainability and Fecundity of Santolina impressa (Asteraceae: Anthemideae)

Santolina impressa (2n = 2x = 18) is an endemic species of Portugal, with restricted geographical distribution. The present study aimed to explore its chromosomal variation in respect to chromosome morphology, meiotic behaviour, and effects on pollen stainability and fecundity. Its karyotype formula was found to be either 12m + 2m^sat + 2sm^sat + 2st (75% of the individuals) or 12m + 2m^sat + 3sm + 1sm^sat (25% of the individuals). Univalents were observed in 29.21% of the meiocytes. Chromosome fragments due to breakage in the chromosome arm were observed in 10 meiocytes only (11.23% of the meiocytes). Chains and rings of trivalents were observed in 14.60% of the meiocytes (one trivalent per meiocyte was observed). Chains and rings of quadrivalents were observed in 21.34% and 11.23% of the meiocytes, respectively, with a range of 0–1 per cell. Nine plants (40.90% of the total) with 2n = 2x = 18 + 2B showed a quadrivalent configuration in diakinesis. Twenty abnormal anaphases with delayed disjunction of the four non-homologous and the two homologous chromosomes were observed. Simple chromosome bridges without fragments and interchromosomal adhesions were observed in 35.95% of the anaphases analyzed. B chromosomes showed bivalent association in diakinesis and their segregation at anaphase I was normal. Pollen was found to be fertile (mean ± s.d. = 89.57 ± 47.14%); the effect of univalent frequency and frequency of abnormal anaphase I on pollen stainability was strong and statistically significant.     

Recovery of COCs from ovaries with high follicle numbers enhances in vitro embryo yield in sheep

The aim of the present study was to investigate the correlation between the number of ovarian follicles and in vitro embryo development and quality in sheep. Sarda ewe ovaries were classified according to the number of follicles: or=8 (High). IVM, IVF and IVC were performed under standard conditions. Cleavage rate and blastocyst development were assessed 48 h after fertilization and on Days 6, 7 and 8 of culture, respectively. Expanded blastocysts were vitrified; blastocoel re-expansion and hatching rates were assessed at 8, 16 and 72 h post-thawing and hatched blastocysts were analyzed with the TUNEL assay. In a subset of thawed blastocysts the incorporation of amino acids was evaluated. The proportion of ovaries varied significantly among the three groups (ANOVA F=12.20, P=0), and more ovaries (59%) were assigned to the Low group than to the Intermediate (28%; ANOVA F=8.19, P=0.009) and High group (13%; ANOVA F=18.63, P=0), (High vs. Intermediate F=6.31, P=0.020). The three groups statistically differed in the proportion of total blastocysts (chi(2)(2)=22.616, P=0.00), of blastocysts produced on Days 6 (chi(2)(2)=6.829, P=0.033) and 7 (chi(2)(2)=6.810, P=0.033), while no difference was found in the proportion of blastocysts obtained on Day 8 (chi(2)(2)=3.874, P=0.144) of culture after fertilization. A higher proportion of total blastocysts was obtained from the High (44%) compared with the other two groups (Low: 28%, chi(2)(2)=22.629, P=0; Intermediate: 33%, chi(2)(2)=7.266, P=0.007), while the Low and Intermediate groups did not statistically differ either in the total blastocyst output (chi(2)(2)=3.384, P=0.066), nor in the number of blastocysts produced on Days 6 (Low: 7%, Intermediate: 9%; chi(2)(2)=0.874, P=0.35), 7 (Low: 14%, Intermediate: 16%, chi(2)(2)=1.256, P=0.26) and 8 (Low: 6%, Intermediate: 7% chi(2)(2)=0.554, P=0.45) of culture. The High group produced a significantly higher percentage of embryos on Days 6 (High: 13%, Low: 7%; chi(2)(2)=6.840, P=0.009) and 7 (High: 21%, Low: 14%; chi(2)(2)=6.806, P=0.009) of culture post-insemination than the Low group. The three categories did not differ in the blastocoel re-expansion (chi(2)(2)=0.095, P=0.95) and hatching rates (chi(2)(2)=0.754, P=0.68) after 72 h post-warming, in the total number of cells per blastocyst (ANOVA F=1.12, P=0.337) and in the (F=0.46, P=0.639) incorporation of amino acids. The number of TUNEL-positive cells per embryo was higher (ANOVA F=4.32, P=0.022) in the Low group compared to the other groups. In conclusion, high ovarian follicle number enhances in vitro embryo output in sheep, but has no effect on blastocyst quality.     

Cytogenetic effects of 241Am in the Allium test

The purpose of this study was to elucidate the genome damage induced by 241Am-irradiation using different parameters of cytogenetic evaluation in Allium-test. The root tip cells test-system for the cytogenetic effects studying was used. 241AmCl3 of different concentrations was used (1.5 x 10(-9)-1.5 x 10(-7) g/l). Water solution-to-plant transfer factor for 241Am was found to be 0.18 +/- 0.04. The internal doses of 241Am accumulated during germination were 0.37-37.00 cGy. The impact of 241Am-irradiation was evaluated on the mitotic index (MI), the yield of aberrant anaphases (AA), the distribution of chromosome aberrations number in cells and the average level of lesion of aberrant cell (LAC). Probably all these parameters are differ in sensitivity to damage factor, but only some changes in MI was revealed. It is supposed, that the absence of any changes in the distribution of chromosome aberrations number in cells and the average level of LAC in 241Am-irradiated cells confirm the absence of significant 241Am-impact on chromosomes, as the alpha-irradiation should cause significant damages in chromosomes. Although solutions of 241Am were high-concentrated, the seedlings didn't accumulate high internal doses. It appears the distribution of 241Am is a significantly heterogeneous hence it is possible the absorbed doses in nuclei can't reach the level necessary for revealing of cytogenetic effects.     

Light microscopical studies of chromosomes in embryos of cod, Gadus morhua L.

The chromosomes in the cells of 1–2-day-old cod embryos were studied using light microscopic techniques. The observations revealed slightly non-synchronous cell divisions, with at least three different mitotic phases present in a 1-day-old embryo. Surprisingly high percentages of abnormal metaphases and anaphases were found in normal-looking embryos. In the 2-day-old embryos, free-lying nuclei were observed in the periblast zone. Chromosome studies of these nuclei revealed a cleavage pattern of the chromosomes which differed from that of the other cells.     

Mitotic inhibition and chromosome displacement induced by estradiol in Chinese hamster cells

We tested diethylstilbestrol (DES) and 17 beta-estradiol as mitotic arrestants to determine their effects on chromosome distribution, spindle microtubules, and the cytoplasmic microtubule complex (CMTC) in the Chinese hamster strain Don. Cytological experiments assessed micronuclei induction, chromosome displacement, and anaphase recovery. Indirect immunofluorescence microscopy with antibody to tubulin and electron microscopy were used to illustrate effects on microtubules. Both DES and estradiol were potent inhibitors of mitosis when applied to cells in vitro. Estradiol induced micronuclei at a greater frequency than did DES. Estradiol-arrested metaphases often contained misaligned chromosomes despite the presence of a bipolar spindle and an equatorial plate. Equatorial plates were not observed in DES-arrested cells. Cells recovered quickly from estradiol exposure upon removal of the steroid. The frequency of abnormal metaphases and abnormal anaphases declined as the recovery period increased. Microtubule experiments showed that DES inhibited spindle assembly and disassembled the CMTC, whereas estradiol, at similar concentrations, arrested mitosis in a manner that allowed spindle assembly. A definite effect on the CMTC by estradiol could not be determined. However, changes in cell morphology were observed. In the presence of estradiol, centrosomes organized microtubules that joined with kinetochores of chromosomes at the equatorial plate as well as with those of misaligned chromosomes. Misaligned chromosomes appeared predominantly at polar regions of mitotic cells. Following drug removal, the pole-oriented chromosomes reoriented at the equatorial plate. The unique arresting properties of estradiol may prove useful in studies of chromosome migration and segregation during mitosis.     

Über das Verhalten eines Ringchromosoms in der Mitose und Meiose von Antirrhinum majus L.

Summary The mitotic and meiotic behaviour of a ring-chromosome in Antirrhinum majus was analysed. 26.5% mitotic anaphases showed bridges demonstrating the occurrence of a crossing-over-like process in meristematic cells. From pachytene studies the ring-chromosome could be identified as chromosome 6.An attempt was made to derive the details of the crossing-over process from the various anaphase configurations in pollen mother cells with a heterozygous ring-rod-bivalent. The observed frequencies could only be brought in approximate correspondance with theoretical values by postulating (i) the occurrence of sister-strand and non-sister-strand crossing-over in certain quantitative combinations, and (ii) an unexplained loss or irricognizability of most double bridges in anaphase I.The frequency of plants heterozygous for the ring-chromosome in progenies after seifing was 16.8%. The rate of chromosome mutations in these progenies was not increased. Chromosomal aberrations resulting from meiotic disturbances in the ring plants are probably lost by gonal elimination of unbalanced chromosome sets.     

Features of the chromocenter structure in Drosophila melanogaster larva cells, mutant for genes C(3)G and NOD

Homolog pairing, chromosome morphology, and chromosome disjunction in the first meiotic division were studied in the oocytes of c(3)G/c(3)G female Drosophila melanogaster at developmental stages 3-4 and 14. It was found that homologs were completely or partly paired in some cells (about 20% in either case). The lengths of chromosomes in +/+, +/c(3)G, and c(3)G/c(3)G cells were at a ratio of 1.0:1.6:2.2. The chromocenters of homozygous cells had an abnormal structure. There was no meiotic block in metaphase 1, and chromosomes only segregated equally in about 80% of anaphases of the first meiotic division. The data obtained correspond to the abnormal variants of the formation of the chromocenter in c(3)G/c(3)G females that could be predicted based on the two-ring structure of the chromocenter. The mechanism of the effect of the homo- and heterozygosity for the hypomorphic mutation c(3)G on the formation of the synaptonemal complex (SC) and crossing over frequency was suggested. In nod/nod homozygous females, asynapsis of pericentromeric regions of homologs was observed in the chromocenter. It was assumed that NOD kinezin is necessary at the last stages of pairing of the pericentromeric regions of homologs and formation of the coordinating bonds between them.     

cis-Dichlorodiammine platinum(II) induced aberrations in mouse bone-marrow chromosomes

The clastogenic effect of cis-dichlorodiammine platinum(II) (cis-platin) on mouse bone-marrow chromosomes has been studied. Cis-platin was injected at 3 different doses. Cells were fixed at different time intervals after treatment. Different types of aberrations together with the percent of mitotic index and frequency of abnormal metaphases were studied. The aberrations observed were primarily chromatid breaks, although isochromatid breaks, interchanges, and multiple breaks were also observed. A dose- and time-dependent effect was observed for both inhibition of mitotic index and frequency of abnormal metaphases. Trypsin-Giemsa staining of bone-marrow metaphase chromosomes from normal mice was compared with the bands of metaphase chromosomes obtained after Giemsa staining of chromosomes from platinum-treated mice and they were observed to be identical. Bands were present up to 120 h and aberrations were also induced in such plates.     

PRINS-labeled knobs are not associated with increased chromosomal stickiness in the maize st1 mutant

In maize, the st1 mutant has been observed to result in chromosomes that stick together during both mitotic and meiotic anaphase. These sticky chromosomes result in abnormal chromosome separation at anaphase. Although the mechanism producing the st1 mutant phenotype is unknown, delayed replication of knob heterochromatin has been implicated in similar phenomena that result in sticky chromosomes. Primed in situ labeling (PRINS) was used to locate the 180-bp knob DNA sequences on mitotic metaphase chromosomes of several maize lines. The chromosomal regions labeled by PRINS corresponded to the reported C bands found in these lines. Additionally, PRINS was used to identify knob-bearing regions in anaphase spreads of a line carrying the st1 mutant and a nonmutant line having a similar number of chromosome knobs. The increase in abnormal anaphase figures in the st1 mutant was not accompanied by an increase in association of knob DNA with abnormal anaphases. Thus, the increase in chromosomal stickiness appears to be due to an increase in stickiness of knob and nonknob chromosomal regions. The mechanism responsible for the st1 mutant, therefore, is hypothesized to be different from that implicated in the other previously described sticky chromosomes situations.     

Nondisjunction in human sperm: comparison of frequencies in acrocentric chromosomes.

Acrocentric chromosomes may be particularly predisposed to nondisjunction because of the frequency of trisomy for these chromosomes in human spontaneous abortions and liveborns. Studies of aneuploidy in human sperm have provided data on only a few acrocentric chromosomes, with evidence that chromosome 21 has a significantly increased frequency of disomy. To determine whether other acrocentric chromosomes have a higher frequency of nondisjunction or if chromosome 21 is anomalous, disomy frequencies for chromosomes 13 and 22 were studied by fluorescence in situ hybridization (FISH) analysis of 51,043 sperm nuclei from five normal men for whom the frequency of disomy for chromosomes 15 and 21 was known. The mean frequency of disomy for chromosome 13 (0.19%) did not differ significantly from that for other autosomes; however, the frequency of disomy 22 (1.21%) was significantly elevated (P < 0.001, Mantel-Haenszel chi(2) test). The G-group chromosomes (Nos. 21 and 22) also showed a significantly increased frequency of disomy (0. 75%) compared to acrocentric D-group chromosomes (viz., chromosomes 13 and 15; 0.15%) (P < 0.001, Mantel-Haenszel chi(2) test) and other autosomes (chromosomes 1, 2, 4, 9, 12, 13, 15, 16, 18, and 20; 0. 13%) studied in the same men (P < 0.001, Mantel-Haenszel chi(2) test).     

Evaluation of genotoxic effects of Apitol (cymiazole hydrochloride) in vitro by measurement of sister chromatid exchange

Apitol, with cymiazole hydrochloride as the active ingredient, is used in bee-keeping against the ectoparasitic mite Varroa destructor. The preparation was evaluated for genotoxicity in cultured human peripheral blood lymphocytes. Sister chromatid exchange, the mitotic index and the cell proliferation index were determined for three experimental concentrations of Apitol (0.001, 0.01 and 0.1 mg/ml). All concentrations significantly (p < 0.001) increased the mitotic index (MI = 7.35+/-0.18%, 8.31+/-0.20% and 12.33+/-0.25%, respectively), the proliferative index (PI = 1.83+/-0.01, 1.84+/-0.01 and 1.88+/-0.02, respectively) and the frequency of sister chromatid exchange (SCE = 8.19+/-1.81, 8.78+/-1.80 and 13.46+/-1.88, respectively), suggesting that cymiazole hydrochloride has genotoxic potential.     

Mating behavior of bucks and does in goat operations under range conditions

The objectives of this study were to characterize the mating capacity of bucks under range condition and assess the effect of mating frequency on flock fertility. Two adjacent flocks of crossbred goats (Criolloxdairy breeds; n=70 does, 5 bucks and 141 does, 4 bucks) were used in this study. The mating period was 16 and 20 days for each flock, respectively, in January and February, 1996. Mating activity of bucks and does was recorded day and night during the first 11 days of the breeding period. The combined data of both flocks showed a strong relationship between number of goats in estrus and the average services of bucks (r=0.95; P<0.01). With an excess of estrous does, bucks on average copulated 9.1 times daily during the first 11 days of the mating period. Considering both flocks combined, does copulated on average 4+/-1.8 times through the estrus period, and they did so with an average of 2.2+/-1.3 different bucks. Sexual activity of bucks was greatest from sunset to midday. Number of services from different bucks did not affect pregnancy rate (81.4% for goats serviced by 1-2 bucks and 77.8% for goats serviced by 3-4 different bucks; chi(2)=0.25; P7 services, respectively; chi(2)=0.67; P0.05). Mean+/-S.D. daily liveweight loss of bucks during the entire mating period was 547+/-197 g. It was concluded that, with buck percentages higher than 3%, the number of ejaculations of bucks is linearly and positively related to number of females in estrus. Also, these findings indicate that neither number of copulations nor number of services from different bucks affected kidding rates.     

Persistence of anaphase abnormalities in fast neutron irradiated tomato

The percentage of abnormal anaphases was determined in rooted leaf and stem cuttings from irradiated diploid and tetraploid tomato plants. It was confirmed that somatic selection is very fast but that even drastic abnormalities, giving rise to abnormal anaphases, seem to be stabilised at a low frequency. Little difference was found between diploid and tetraploid lines or between different genotypes. A considerable part of the anaphase abnormalities found are considered to have arisen de novo.     

Lectin and mitotic activities in root meristems of winter wheat under oryzalin effect

The effect of oryzalin, a microtubule polymerization inhibitor (10 MM), on lectin and mitotic activities (mitotic index and duration of mitotic phases) was studied in unhardened (23 degrees C) and hardened (7 days, 2-3 degrees C) winter wheat seedlings. Three wheat cultivars differing in their frost tolerance were compared. Oryzalin treatment (3 h) decreased activity of soluble lectins, increased activity of cell wall lectin mitotic index. Under these conditions, prolongation of anaphases and disappearance of telophases were detected. Plant hardening reduced the sensitivity of cell wall lectins and mitotic activity to the cytoskeleton inhibitor due, presumably, to the appearance of cold-stable microtubules. Plant growing and hardening with oryzalin stopped mitoses and caused the appearance of polyploid cells and cells with micronuclei. These abnormalities were preserved after hardening. The results obtained demonstrate an important role of microtubules in adaptation of plants to low temperature.