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 Hryszphe is, in general, similar to that in 
the brown alge. 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 Lz/éace@ and other monocotyledo- 
nous species, as well as a few dicotyledonous plants such as He/leborus 
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- 
borus fetidus 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 first 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 ina 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 
