MITOSIS IN POLLEN MOTHER-CELLS. + II 



mature, the centrosphere remains as a much flattened disc closely 

 applied to the nuclear membrane. 



From the foregoing it is clear that, although differing much in detail, 

 the karyokinetic process in Erysiphe is, in general, similar to that in 

 the brown algae. At our present state of knowledge, it is difficult to 

 explain all the minor differences or to form an estimate of their 

 relative importance. 



MITOSIS IN POLLEN MOTHER-CELLS. 



The spore mother-cells of certain Liliacece and other monocotyledo- 

 nous species, as well as a few dicotyledonous plants such as Helleborus 

 and Podophyllum, have become classical objects for cytological study, 

 and in these genera the mitotic process is now as well understood as in 

 any other angiosperms. The following discussion of the first two 

 nuclear divisions in the spore mother-cells of higher plants is based 

 upon the author's own investigations made upon Lilium martagon, 

 L. candidum, Fritillaria persica, Tradescantia virginica, Helle- 

 borzis foetidus and Podophyllum peltatum. 



THE FIRST OR HETEROTYPIC MITOSIS. 



RESTING NUCLEUS AND DEVELOPMENT OF CHROMATIN SPIREM. 



Soon after the last nuclear division in the archesporium, or spore- 

 bearing tissue, which gives rise to the pollen mother-cells, the latter 

 begin that period of growth so characteristic of spore mother-cells pre- 

 viously to the fh'st mitosis. The nucleus is relatively large with a 

 sharply defined membrane, and contains a fine linin network, in which 

 the chromatin granules are held, and one or more nucleoli. The 

 nucleolus may lie in a colorless, spherical cavity, which seems sharply 

 circumscribed. The chromatin appears in larger and smaller granules, 

 which are, as a rule, regularly distributed in the linin thread. The 

 cytoplasm presents a uniform netlike structure (Fig. 5, A). This is 

 the typical structure of a pollen mother-cell. 



With further growth of the nucleus, the chromatin granules increase 

 in size, probably through the union or aggregation of the smaller 

 granules, while at the same time the linin thread contracts and shortens. 

 In this stage the linin net consists of a complicated spirem or thread 

 with short turns. The chromatin granules have attained a more uni- 

 form size, and lie more regularly distributed in the linin thread (Fig. 

 5, B). This contraction of the linin thread and fusion of the smaller 

 chromatin granules continues, so that the nuclear thread, which later 



