THE MALE GAMETOPHYTE 135 



even coiled or twisted, as in Tradescantia (Coulter and 

 liose 14 ) ; or the cytoplasm of the spindle-shaped generative cell 

 may taper into elongated whip-like filaments that more or less 

 encircle the tube-nucleus, as in Eichhornia (Smith 39 ). In 

 Erythronium Schaffner 55 found that the generative nucleus is 

 larger than the tube nucleus and is surrounded by a densely 

 staining amoeboid-form mass of cytoplasm. It is altogether 

 probable that the size and form of free generative cells varies 

 with age and external conditions, so that they may be relatively 

 large or small ; or spherical, lenticular, spindle-shaped, or ver- 

 miform in the same species. It is very common to find them at 

 first spherical and later lenticular, as has been frequently ob- 

 served in Lilium. 



In Lilium tigrinum Chamberlain 32 often found a small cell 

 cut off by the microspore before the appearance of the tube and 

 generative nuclei, and the same cell was noted after the division 

 of the generative nucleus (Fig. 63). A similar cell was found 

 by Smith 39 in Eichhornia crassipes and by Campbell 43 in Spar- 

 ganium simplex. It is suggestive of a true vegetative or pro- 

 thallial cell, two of which so commonly occur among the Gym- 

 nosperms ; but the phenomenon is too unique as yet among 

 Angiosperms to deserve more than a mention. 



The tube-nucleus usually increases much in size, and under 

 certain conditions has been found to fragment, as in Lilium, in 

 which Chamberlain 32 found four and in one case eight tube- 

 nuclei ; in Eichhornia, in which Smith 39 found two tube-nuclei 

 in half the pollen-grains examined; in Hemerocallis, in which 

 Fullmer 44 reports the frequent occurrence of two to six tube- 

 nuclei ; and in Asclepias, in which Frye 56 observed a fragment- 

 ing nucleus. This phenomenon is doubtless not uncommon in 

 certain conditions of nutrition. 



The generative nucleus or cell may divide in the pollen- 

 grain, even long before dehiscence, as in Sagittaria (Schaff- 

 ner 31 ) ; or the generative cell may pass into the tube before 

 division, sometimes not dividing until immediately before fer- 

 tilization. The time of this division seems to hold no relation 

 to the great plant groups, and may be variable in the same genus 

 or even species. For example, in Lilium tigrinum it often 

 takes place in the grain, but in L. phi/adelphicum rarely so; 

 and in this last species it may occur either in the grain or at 

 10 



