122 Taylor — A Morphological and Cytological 



nears maturity, shows three thin lines which after shedding 

 permit the emergence of the pollen tube. The mature pollen 

 grain has but two nuclei, the tube and generative nuclei, and 

 the latter remains undivided till after the pollen grain has been 

 shed. The appearance of the pollen grains of the various spe- 

 cies is similar, size being the only evident difference (Fig. 27). 

 The pollen grains of anthers in female flowers more often showed 

 abnormalities than in anthers of male flowers, and showed de- 

 generation as anthesis approached. 



Pollen and Somatic Divisions in Other Species 



The history of pollen development as given for Acer negundo 

 is typical in a general way of the genus. The differences ob- 

 served in other forms are for the most part those necessary to 

 effect the distribution of the larger number of chromosomes. 



Amitosis was the most common form of nuclear division in 

 the tapetum cells, but mitotic division was observed in other 

 forms than Acer negundo, notably Acer pseudo-platanus. There 

 was no special differentiation of the cytoplasm near the spindle 

 in Acer negundo, but in Acer saccharum and Acer platanoides 

 a denser zone was present, slightly removed from the spindle 

 and surrounding it. In Acer rubrum such a dense area was pres- 

 ent in a pronounced form, frequently assuming a unilateral or 

 unipolar position (Figs. 29, 30). This appearance is hardly 

 likely to be an artifact, for the general fixation of the cytoplasm 

 and of the chromatin is quite good. In the forms with high 

 chromosome numbers there was naturally a greatly increased 

 complexity of the spireme at all stages, and especially was this 

 true of Acer rubrum. A complete discussion of certain peculi- 

 arities of this form are given later, but it is best mentioned 

 here that as strepsinema approaches there appears a much more 

 decided twiscing of the spireme than was found in Acer negundo 

 (Fig. 28). This was true to some extent of the other forms. 

 The extreme complexity of the spireme mass prevented a com- 

 plete study of Acer rubrum. Certain material of this species 

 showed a great deal of abnormality in the pollen grains, described 

 later, and in addition to these structural variations in some 

 cases delayed division of the nucleus into tube and generative 

 nuclei. Division of the generative nucleus within the grain was 

 not observed, either as a normal or as an abnormal occurrence. 



