XI] STRUCTURE OF POLLEN 97 



The completion of the proof of homology is seen in the 

 fact that some Gymnosperms have motile ciliated sperma- 

 tozoids in their pollen-tubes, just like those which occur 

 in the antheridia of Cryptogams ; and even in higher 

 flowering plants e.g. Lilies curved, writhing bodies of 

 this kind occur. 



Returning to the pollen-grain itself, then, we have 

 seen that, microscopically examined, a typical pollen- 

 grain, fully mature, is found to consist of protoplasm 

 invested by a cell- wall ; it is practically an isolated cell, 

 though in most cases both the investing wall and the 

 protoplasmic contents exhibit complexities which show 

 that we are here dealing with what is more than a simple 

 single cell. 



In the first place, the investment consists in the 

 majority of cases of at least two membranes or layers ; 

 an outer, firmer, and especially protective layer the 

 exine diversified by the most varied sculpturing, such 

 as papillae, spines, warts, ridges, honeycombing, and all 

 manner of combinations of these and other markings ; 

 and an inner, thinner, and more yielding layer the 

 intine devoid of irregularities. In many cases, again, 

 the exine has oily substances smeared on it, and in 

 some grains there are definitely placed, circumscribed, 

 thin areas, looking as if a minute pair of compasses had 

 been used to incise a circle nearly through the exine, at 

 which the intine can easily be pressed through by the 

 swelling of the contents. In the latter case the excised 

 circles act like lids, and are pushed off as such (e.g. 

 Passion-flower, Gourd, Morina, &c.) in the process of 

 germination. These thin areas, or windows, are places 

 for emergence of the pollen-tubes, and remind one forth- 

 with of the very similar thin areas which facilitate the 

 emergence of the germinal hyphse in many fungus-spores 

 w. in. 7 



