976 RECORD OF CURRENT RESEARCHES RELATING TO 



BOTANY. 



A. GrENEBAIi, including Embryology and Histology 

 of the Phanerogamia. 



Division of the Nucleus in the PoUen-Mother-cells of Trades- 

 cantia.* — According to Baranctzky, the mother-cells of the pollen of 

 some species of Tradescantia, especially T. virginica, pilosa, discolor, 

 suhaspera, and zebrina, afford a remarkably good illustration of the 

 mode of division of the cell-nucleus. Almost as soon as division 

 commences they separate entirely from one another ; and it is only 

 necessary to crush the anthers in water under the cover-glass in order 

 to get readily the cells swimmiug about separately in a state of division, 

 and hence to observe the processes on all sides. Owing to the thin- 

 ness of the protoplasm, the cells will remain uninjured for hours in 

 river or spring-water without undergoing any material change ; after 

 a longer time the cell-contents contract without any formation of 

 vacuoles. The nucleus can be readily made out, even when the cells 

 lie only in water, from its density and sharp outline. The descriptions 

 of the process by previous observers are in certain points incorrect, 

 probably because it has been observed in salt or sugar solutions, in 

 which the nucleus is not nearly so clear, instead of simply in water. 



Very young pollen-mother-cells are filled with a moderately dense, 

 finely granular protoplasm. The large nuclei, which are considerably 

 denser, appear also to be finely granular ; they have no membrane- 

 like outermost layer. The behaviour of the nucleoli is not altogether 

 clear. In T. zebrma there appears to be always a large nucleolus, 

 while in the other species none was clearly visible, and in some it was 

 altogether unrecognizable. Probably they are always present when the 

 nucleus is in a state of rest. The division now proceeds in the way 

 described by Hanstein. The dense parts of the nucleus, which at first 

 appear like fine granules without definite form, increase in size, and 

 gradually assume the form of short rods inclined in different direc- 

 tions, and separated by a sparse clear matrix. The appearance is, 

 indeed, somewhat as if the nucleus were full of bacteria. A nucleolus 

 can now be detected in the nucleus, usually with definite outline. The 

 rods appear to be of different lengths, but their terminations are not 

 readily made out ; they become thicker, while their number decreases, 

 and the nucleus becomes gradually denser and less transparent. At 

 one stage the appearance is more that of uninterrupted threads than of 

 rods or granules, and this is probably the case throughout. 



As the process advances, the rods and threads, though increasing 

 in size, never lose their sharp outline, while the quantity of the inter- 

 mediate matrix diminishes considerably. The denser portions of the 

 nucleus are in its periphery, and in immediate contact with the pro- 

 toplasm of the cell ; and this causes the nucleus to lose its smooth 

 outline, and to bulge out with a number of protuberances, and it pos- 

 sesses this mamillated form when the rods have attained their full 

 size. 



The mass of the protoplasm of the cell is at first indistinctly finely 

 * 'Bot. Zeit.,' xxxviii. (1880) p. 241. 



