100 DuGGAR : Developmkxt of the Pollen Gralv 



plasm shows the effect of a general radial arrangement, from which 

 there is soon differentiated the complex spindle characteristic of 

 dicot}'ledons (Fig. 31). 



In the dau<ihter nuclei it is ac^in evident that the chromosomes 



. ...^. ....^.^. .V, X.. ..j^ 



first become Irregular and sHghtly fused, as in Fig. 31, and grad- 

 ually the fusion continues, or by some process it is returned to one 

 or to sev^eral chromatic masses. Where there are several large 

 masses first formed, the fusion of these into a single mass is not 

 always evident ; but eventually one prominent nucleohis results, 

 as in Fig. 32, thus agreeing with the condition found in the first 

 division. Tliis gradual mergence of the chromatin mass into 

 characteristic forms has been carefully followed, and various stages 

 in the return of the chromatin from the chromosome state to that 

 of the large nuclear masses are shown in Fig. 33. This mass is 

 .at first irregular in outline, but in time this irregularity is lost. 

 One or more linin attachments persist, and everything indicates 

 that the nucleolus of the microspore nucleus has thus resulted 

 from the direct or indirect fusion of chromatic material used in 



division. 



IJie Microsjyof'e 



The forming microspores become invested with walls of their 

 own, even the cell plates of previous div^Isions disintegrating with 

 the general wall of the mother-cell. Wlien first set free, the micro- 

 spore IS somewhat elliptical in form, the nucleus small, and the 

 nucleolus relatively large. It remains in the resting condition for 

 jsome time, and undergoes a period of growth, during which time 

 the tapetal cells disintegrate rapidly. Until the divisions in the 

 pollen mother-cells are begun the tapetal cells have undergone no 

 disintegration, although becoming granular in appearance and 

 staining deeply. The mature microspore is invested with a very 

 thick wall, and owing to lack of stages and some difficulties with 

 fixing, I have not been able to study the division of the microspore 



nucleus. As a result of this division, however, the pollen grain 

 contains a larger vegetative nucleus, and a smaller more chromatic 

 generative nucleus. The latter is not sejoarated in a daughter-cell 

 by a permanent cell wall, for In the mature pollen grain it Is a free 

 nucleus. Division of the generative nucleus does not take place 

 previous to germination. 



