The individuality of the chromosomes and their serial arrangment, etc. 121 



In the nuclei of cells situated in the outer layers of the root cap and 

 in still older portions of the root the chromosomes cannot be identified. 

 In the dying cells of the root cap the nuclei decrease in size, the nucleoles 

 become smaller and the nuclear cavity becomes filled with a dense granular 

 substance which stains quite uniformly. 



The Sporogenous Tissues. 



Everywhere in the cells of ovary walls, styles, stigmas, filaments, 

 tapetum, and sporogenous tissue the chromosomes are recognizable in 

 the resting nuclei. They are especially conspicuous in the tapetal nuclei 

 as shown in figure 3, in the nuclei of the filaments (fig. 7 6), and in the 

 nuclei of cells in the walls of the ovary (fig. 2). These nuclei are as a 

 rule smaller than those of the root tips and accurate counts of such large 

 numbers of chromosomes is not possible, but it is clear that the number 

 of chromatic units does not exceed the chromosome number and that 

 there is never a completely reticulated condition of the chromatin. 



The resting nuclei in the archesporial cells of the young stamens 

 range from 4,0 (i to 5,25 (.i in diameter and are hence slightly smaller 

 than the root tip nuclei. In the prophases of division, however, the 

 nuclei often increase in diameter to 8,0^ or 8,25 a which is slightly greater 

 than the diameter of the nuclei in the root tip during the same stage. 



A cross section through a loculus at the stage of the early divisions 

 of the archesporial cells shows that there it consists of a single outer 

 layer of cells forming the wall, two rows of tapetal cells, and a single 

 circle of wedge-shaped sporogenous cells. In such a section four or five 

 sporogenous cells are usually shown. They are closely crowded together 

 and are wedge-shaped in cross section with the apices in the center. The 

 diameter of the entire cylinder of spore producing tissue at this stage 

 is about 17 to 20 it. 



The Heterotypic Division. 



When the spore mother-cells are fully formed and are passing into 

 synapsis, five or six appear in a cross section of a loculus and the diameter 

 of the whole cylinder of cells is from 22 to 25 ii. In the resting nuclei 

 of the spore mother-cells the chromosomes are situated at the periphery 

 and appear as shown in figure 19. In the presynaptic stages the nucleus 

 enlarges to about 6,5 or 7,5^ in diameter. The chromosomes draw 

 away from the nuclear membrane appearing as in figure 20. They are 

 now fusiform, which is due apparently to a sort of spinning out of their 



