The 30th of July was the day of the full bloom. 



thickened in certain spots. At late anaphase the 

 two chromatin groups are connected by means 

 of the chromatin threads. The second reduction - 

 division proceeds in the same manner as the 

 first. The pollen tetrads are improperly 

 formed; they may contain two normal-sized 

 cells, the third may be giant, and the fourth, 

 dwarf. In addition, various other phenomena 

 were observed: pentads, binuclear and mono- 

 nuclear tetrad cells, multicellular tetrads, and 

 one to several nuclei of various sizes (some- 

 times tiny) in each cell. 



The works of Goodspeed (1929 to 1937) occupy 

 an important place in the annals of X radiology. 

 For a decade he and his colleagues studied the 

 effects of X rays on the genus Nicotiana . He 

 began his experiments in 1927 with N. tabacum 

 (1929). All opened flowers and buds were first 

 removed from the inflorescence. The seeds of 

 the irradiated plants yielded 1040 plants of which 

 40 were subjected to detailed cytological analy- 

 sis. Two plants of the 40 showed a loose inter- 

 connection of the chromosomes in the diakinesis, 

 and the number of microcytes in the tetrads was 

 greater than the usual four. During interkinesis 

 lagging chromosomes could always be observed 

 and in the second division there were invariably 

 less than 24 chromosomes participating. Four 

 of the plants had 23 bivalents and a single uni- 

 valent on their metaphase plate; three plants 

 had 24 bivalents and one univalent; the eighth 

 one had 22 bivalents and one univalent or 23 

 bivalents. Consequently, the last eight plants 

 represented simple meiotic aberrants. 



Chromosomal fragmentation was observed in 

 five other plants. In one of the plants an extra 

 chromosome of small size appeared on the 

 somatic plates. During reduction -division two 

 of these plants had 22 bivalents and 23 bivalents 

 plus a tiny body; a third had 23 bivalents and a 

 single fragment. It is interesting that all the 

 fragments were identical in shape and size. 

 Usually these fragments place themselves be- 

 tween the bivalents and divide during the second 

 division. Sometimes they divide during the 



first division and their halves migrate to 

 opposite poles. It frequently happened that this 

 division was unequal. 



In his joint work with Avery (Goodspeed and 

 Avery, 1930), Goodspeed describes in detail 

 the reorganization of chromosomes under the 

 influence of irradiation. The somatic set of 

 chromosomes consisted of 50 units and during 

 the first division there were 24 or 25 units. 

 The authors explain this difference by the 

 alternating appearance of quadrivalents. The 

 formation of rings is characteristic of irradiated 

 tobacco since this phenomenon is not observed 

 in normal tobacco. The quadrivalents arise 

 due to the conjugation of two bivalents, which 

 testifies to the homology of all four chromo- 

 somes. However, one of these bivalents is 

 simple, whereas the other has a complex origin, 

 actually being a trivalent, consisting of two 

 large elements connected by a third, which is in 

 the center. This small component arose from 

 the division of a small segment of one of the 

 partners of the bivalent, which afterwards joined 

 to the other partner. In this fashion X rays in- 

 duced a basic reorganization of the chromo- 

 somes. In another case, a very complex ele- 

 ment was observed in the pollen mother cells. 

 Apparently, the opposite ends of a single, whole 

 chromosome fused, which led to the formation 

 of a "bridge" at anaphase and to the appearance 

 of fragments. In a third case, a new type of 

 reorganization was observed: the permanent 

 appearance of a tiny fragment. A genetic 

 examination leads us to believe that this addi- 

 tional phenomenon is due to a translocated seg- 

 ment from a non -homologous chromosome. 

 The composition of the nucleus in this plant is 

 complicated by the appearance of five complex 

 configurations, one of which is obviously a 

 trivalent. A fourth plant had 24 elements, of 

 which two were trivalent. In the chromosome 

 set of the pollen mother cells there were 28 

 elements, 25 of them bivalent, two univalent, 

 and one a fragment. One bivalent had very 

 small dimensions and was weakly colored by 

 hematoxylin; apparently, it was formed from 



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