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CURRENT LITERATURE.
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Histology.
Gates, R. Ruggles. A preliminary account of the rneiotic phenomena 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."
