Karl Sax

A Cytological Monograph of the American Species of Tradescantia

1. Autopolyploidy, aneuploidy, segmental interchange, fragmentation, and differences in chromosome size were found in the nine genera and thirty-two species of the Commelinaceae which were investigated cytologically. 2. Chromosome pairing has been studied in five diploid species of Tradescantia. The average number of chiasmata per bivalent is found to be fairly constant for any one plant but varies widely from plant to plant, even within the same species. 3. Pollen sterility varies from 82 to 98 per cent among the diploid species and ranges from 71 to 89 per cent among the autotetraploids. 4. An extensive cytological survey of the species allied to T. virginiana shows them to be made up largely of (1) diploid races and species, mostly southern, and (2) autotetraploid races and species, mostly northern in distribution. 5. The autotetraploids have roughly twice the average radius of distribution of the diploids. 6. Terminal chiasmata and median attachment constrictions, conditions favoring autotetraploidy, are discussed. 7. Autotetraploid Tradescantias are slightly larger and have longer blooming periods than the related diploids, but can be distinguished from them only cytologically. 8. Cytological conditions favoring autotetraploidy have brought about the independent evolution in different species of Tradescantia of vigorous autotetraploid races. 9. Five artificial interspecific diploid hybrids were produced experimentally and studied cytologically. Pairing in these hybrids is regular although the chiasma frequency is slightly lower than the average of the parents. Pollen fertility varies from 40 to 73 per cent. The evidence therefore indicates that structural changes have played little or no part in the immediate differentiation of these species. 10. Natural hybrids are discussed and illustrated by examples. 11. Certain of the internal and external barriers to interspecific hybridization in the American Tradescantias are enumerated. 12. The various consequences of hybridization under different conditions are described. 13. The cultivated Tradescantias are compared with the wild species from which they were ultimately derived and their cytological differences are compared statistically.

The stimulation of plant growth by ionizing radiation

Abstract The stimulation of plant growth by exposing seeds or growing plants to low doses of ionizing radiation, or by the use of radioactive fertilizers, has been reported by many investigators during the past 65 years. There is critical evidence that ionizing radiation may stimulate plant growth at certain stages of development and may induce earlier flowering. It can also stimulate lateral bud development, presumably by auxin inactivation. These stimulating effects are usually of small magnitude and are often not reproducible, although in individual tests the differences may be statistically significant. The evidence for increased yields of crop plants is less conclusive. Large increases in yields of crop plants by irradiating seed, soaking seed in radioactive solutions, or by the use of radioactive fertilizers have been reported from the U.S.S.R. Similar tests in the U.S. have given only negative results.

Development of the Male Gametophyte in Tradescantia

1. Development of the male gametophyte in Tradescantia seems to be correlated with a polarity established during the meiotic divisions. The spindle of the first microspore division is almost invariably oriented across the short axis of the cell, and the dividing nucleus is nearer the cell wall formed during meiosis. The nucleus formed near this inner cell wall becomes the elongated generative nucleus which divides in the pollen tube to form the two male gametes. The other cell, surrounded by most of the cytoplasm in the microspore, becomes the vegetative or tube nucleus and remains in the resting condition. Development is associated with a movement of the microspore nucleus before division and the formation of characteristic vacuoles. 2. Non-disjunction in segmental interchange species results in chromosome deficiencies in the dyad nuclei, but these nuclei divide normally at the second division, even though they are deficient for a chromosome segment. But the deficient one-nucleate microspores do not develop further. It is suggested that development of the microspore is dependent on a full complement of gene products which pass into the cytoplasm at the preceding division. 3. A brief chronology of meiosis and microspore development is presented. About two weeks elapse between early meiotic prophase and the formation of mature pollen grains in Tradescantia reflexa.

An X-ray analysis of progressive chromosome splitting

SummaryThe chromosomes of a diploidTradescantia became effectively double with respect to X-rays at a time between 30 and 23 hr. before metaphase of the first pollen grain mitosis. During this period there were no differences to be observed between the loci of chromatid and chromosome breaks, which indicates that the splitting of all the chromosomes in any one nucleus, anda fortiori of any single chromosome, is accomplished in a considerably shorter period.The analysis of fifteen microspores in which chromatid and chromosome breaks were discovered coincidently supplies some evidence for the belief that the splitting of single chromosomes is progressive, beginning near the centromere and proceeding distally. The observations, however, as might be expected with such a small sample were of no real statistical significance.The proximal regions of the chromosome arms show a higher frequency of breaks, of both types, than the distal portions. This localization of breakage is attributed to the existence of a greater torsional stress in the proximal regions.Either progressive splitting or differential stress would account for the localization of crossing-over, though the former seems in better agreement with the phenomenon of interference.

Structure of Meiotic Chromosomes in Microsporogenesis of Tradescantia

1. The chromatids of Tradescantia chromosomes are coiled together in a single coiled chromonema at early metaphase of the first meiotic division. At late metaphase the two coiled chromatids in each homologue separate, forming a bivalent with four separate coiled chromatids. The primary coils disappear during interphase, and at the second meiotic division only the secondary coils are found. 2. The primary coils of meiotic chromosomes are assumed to be caused by the contraction or compression of the two flexible chromatids within the chromosome pellicle without rotation of the chromatids. 3. The direction of the primary coils is apparently at random for the homologous chromosomes of each bivalent. Within each homologue the direction of coiling may change at the fiber constriction, but there is a tendency for the coiling to be in the same direction on both sides of the fiber. The direction of coiling may also change at an interstitial chiasma. 4. We have found no evidence that the meiotic chromatids are split in preparation for the division in the microspore.

CYCASIN: RADIOMIMETIC EFFECT.

Cycasin, methylazoxy-methanol-β-glucoside, a carcinogenic compound that occurs naturally in cycad plants, induces chromosome aberrations in the root-tip cells of Allium cepa. Germination and growth in an aqueous solution of cycasin at a concentration of only 3 percent of that found in Cycas circinalis induced as many aberrations in chromosomes as did about 200 roentgens of gamma irradiation.

A Cytological Analysis of Self-Sterility in Tradescantia

1. Germinating pollen grains of Tradescantia are described as they appear in aceto-carmine smears of pollinated styles. 2. Many species of Tradescantia are self-sterile. 3. Compatible and incompatible pollination can be distinguished cytologically. The pollen grains germinate normally in the latter, but their growth is inhibited. 4. Cross-sterile pollinations cannot be distinguished quantitatively nor qualitatively from self-sterile pollinations. 5. Natural populations of diploid tradescantia can be divided into intra-sterile, inter-fertile classes on the basis of pollen tube behavior. 6. The number of these classes in natural populations is found to be large. 7. The main facts revealed by this purely cytological examination are in line with the oppositional factor hypothesis although there is disagreement on a few minor points.

Mechanism of Mitosis in Pollen Tubes

The type of division found in pollen tubes varies with different genera, species, and among pollen tubes of the same culture. Division of the generative nucleus is generally normal in Scilla, Bellevalia, and Zephyranthes, while atypical divisions are frequent in pollen tubes of Tradescantia, Gasteria, and Lilium. When spatial relations are adequate the chromosomes form on an equatorial plate and the anaphase chromosomes pass to their respective poles in unison; but large chromosomes in a narrow pollen tube divide without formation of an equatorial plate, and the daughter chromosomes appear to act as independent units as they pass irregularly to the poles. These divisions are dependent upon the action of the centromeres and a spindle mechanism. Very long spindles are often found in divisions of the generative nucleus, and in Bellevalia they are curved or U-shaped when the division occurs in the pollen grain or in bulges in the pollen tube. The relation of these abnormal divisions is discussed in relation to the mechanics of mitosis.

THE STIMULATION OF PLANT GROWTH BY IONIZING RADIATION

The stimulation of plant growth by exposing seeds or growing plants to low doses of ionizing radiation, or by the use of radioactive fertilizers, was reported by many investigators during the past 65 years. There is critical evidence that ionizing radiation may stimulate plant growth at certain stages of development and may induce earlier flowering. It can also stimulate lateral bud development, presumably by auxin inactivation. These stimulating effects are usually of small magnitude and are often not reproducible, although in individual tests the differences may be statistically significant. The evidence for increased yields of crop plants is less conclusive. Large increases in yields of crop plants by irradiating seed, soaking seed in radioactive solutions, or by the use of radioactive fertilizers were reported from the USSR. Similar tests in the U. S. have given only negative results. (auth)

THE EFFECT OF CHRONIC GAMMA IRRADIATION ON APICAL DOMINANCE OF TREES

Abstract Cherry and apple trees subjected to low doses of gamma irradiation (average of 16 and 34 r/day) for ten years produced atypical branch development and reversal of dominance which could be attributed to the decrease or inactivation of auxin resulting from the irradiation.