1444 



A TEXTBOOK OF THEORETICAL BOTANY 



however, to note that in Zostera, where the pollen grain itself is a tube 

 about 2 mm. long and no thicker than a normal pollen tube, division 

 appears to be quite regular. It is difficult to believe that there can be any 

 fundamental difference between mitosis in the pollen grain and in the 

 pollen tube, especially as the process may begin in the grain and be finished 

 in the tube. The observed distinctions may be safely attributed to physical 

 conditions, such as spatial restriction, and perhaps to the fact that the cyto- 

 plasm in the pollen tube is in a very active state of circulation. 



There is no doubt that in some species spindle fibres cannot be seen in 

 divisions in the tube, though they have been clearly demonstrated in others. 

 The fact that colchicine interrupts chromosome movement in these divi- 

 sions in the same way that it affects normal mitoses, argues in favour of the 

 mechanism being the same, even when the spindle is not visible. A meta- 

 phase plate can be formed only when the tube is wide and the chromosomes 

 are small. Otherwise only the essential of a plate is possible, namely the 

 arrangement of the kinetochores in one plane, while the bodies of the chro- 

 mosomes may lie in all directions. 



Of much greater importance is the problem of the fate of the cytoplasm 

 of the sperm cells and the question of whether it survives to take part in 

 fertilization, for this may have a genetical significance. Most of the older 

 observers thought that by the time the male nuclei entered the embryo sac 

 they were naked and that their cytoplasm had disappeared. Modern obser- 

 vations have, however, tended generally to confirm the continued existence 

 of cytoplasm, at least up to the anaphase in division of the generative nucleus. 

 It has even been assigned a special importance at this stage as providing a 

 stable enclosure for mitosis and preventing the disturbance of the chromo- 

 somes by cytoplasmic streaming in the tube. Many observations go further 

 than this, and confirm that a zone, of somewhat modified cytoplasm, hyaline 

 and structureless, which has been called the paragenoplast (Fig. 1325), 

 continues to surround the sperm nuclei after division and indeed into the 

 embryo sac. 



;;si«/S£j'i&A&i"''-'""''-'''-"^'^'''^'' 







B 





Fig. 1325. — Vallisneria spiralis. A and B, Portions of pollen tubes from the st\le 

 showing complete generative cells and in A the tube nucleus to the left. (After 

 Wylie.) 



A number of descriptions refer to the sperm nuclei as " vermiform " 

 and in some cases the chromatin does appear to be spiralized, while still in 



