151 

 CURRENT LITERATURE. 



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Histology. 



Gates, R. Ruggles. A preliminary account of the rneiotic pheno- 

 mena in the pollen mother-cells and tapetum of lettuce (Lactuca 

 sativa). Proc. Boy. Soc. London, B. 01 : 216—223. 2 figs. 1920. 



The material under investigation is an improved variety of lettuce, and 

 a "rogue " arising from it. The two plants agree in details of reduction, and 

 bhow certain striking deviations from the usual normal behavior of pollen 

 mother-cells. In the first place, every gradation was observed between 

 tapetum and mother-cells. The tapetum become hi-, and in some oases 

 tetra-nucleate. At the mitosis of the binucleate condition a few instances of 

 typical synapsis have been observed. This is taken to mean, not that tapetal 

 cells are degraded spore mother-cells, but that synapsis may be merely a 

 physiological "' phenomenon of the nucleus which might be directly induced 

 in any diploid cell if it could be placed under the proper conditions." It is 

 believed that synapsis has never before been described for other than spore 

 mother-cells. 



In mother-cells during the synapsis stage, extrusion of chromatin 

 material occurs when the nuclei lie so for to one side of the cell that they 

 touch the cell walls. At diakinesis there are nine bivalent chromosomes, 

 among which constant differences in length are found, some being very long, 

 some intermediate, and some almost spherical. Just before the bivalents 

 begin to shorten and thicken, the constituent chromosomes of many are 

 found to be variously looped or wrapped about each other, in some cases so 

 intimately as to suggest than an interchange of segments may occur when 

 they break apart. This is thought to be the first time such a process of 

 chromosome fusion has been described for plants, though the appearance has 

 been figured by other investigators. 



A few cases of what appeared to be end to end fusion of bivalents was 

 observed, making eight instead of nine pairs of chromosomes in the nucleus. 

 Later, when the chromosomes are arranged on the equatorial plate, all the 

 bivalents become so greatly condensed that they are practically spherica. 

 And here also is observed a tendency for one or two pairs of bivalents to 

 coalesce, resulting in eight or even only seven distinct chromatin masses. 

 All gradations of coalescence are found between this and nine distinct 

 bivalents. This feature also is thought not to have been described before, 

 either for plants or animals. The suggestion is made that such a fusion of 

 bivalents " furnishes a possible basis for the phenomenon of partial coupling 

 or repulsion, apart from the ' crossing over ' phenomena, which are based on 

 relations between the two members of a pair of chromosomes in their earlier 

 post-synaptic stages." 



