13 



divisions produce a mass of sporogenous cells, each of 

 which apparently without further division developes into a 

 pollen mother-cell (Fig. 8). 



Prophases. — Selecting a sporogenous cell to illustrate 

 the heterotype form of mitosis, we find a state of more or 

 less constant duration reached after division of a mother- 

 nucleus and the starting point of the succeeding division in 

 the daughter-nuclei (Fig. 9). This, the so-called resting 

 stage of the nucleus is characterised by a fine reticulum of 

 hnin, upon which the chromatin is deposited in the form of 

 minute granules termed chromomeres— Weismann's Ids. 

 These are considered to be aggregations of still smaller and 

 ultra-microscopic particles variously named. The network 

 gradually becomes more prominent, increases in staining 

 power, and larger irregular masses of chromatin known as 

 karyosomes or net-knots (Fig. 10) appear at intervals along 

 the hnin strands. These chromatin depositing centres 

 when found to equal in number the specific number of 

 chromosomes characteristic of the plant, have sometimes 

 been termed " prochromosomes."^^ 



The chromosome is a thickened segment of the 

 nucleus spirem, and consists of a linin basis in which the 

 chromatin lies imbedded (Fig. 15). One or more nucleoh 

 are present, usually free from the network, the thickening 

 walls of the net lose their reticulate appearance and change 

 into a more or less continuous thread or spirem (Fig. 10), 

 at least ends of the spirem are very difficult to find if 

 present.^ 



Some observers ^' ^ describe for some pollen mother- 

 cells two spirem threads lying side by side, supposed to 

 represent the male and female parts of the chromatin ; 

 others^ that the thread is single but undergoes a longi- 

 tudinal split, which, however, closes up, and later segmen- 



