96 THE POLLEN-GRAIN A SPORE [CH. 



of cells, the development of the pollen mother-cells and 

 their division into fours, and the nutrition of the young 

 pollen-grains, their structure and germination, give us 

 a number of fixed points for comparison which enable 

 us to explain in detail the meaning of all the transient 

 rudimentary structures referred to. 



The formation of the pollen-forming mass and its in- 

 vesting cells is in reality a process of development of 

 a sporogenous tissue i.e. a tissue which gives origin to 

 spore mother-cells and their progeny the spores, and the 

 student can experience little difficulty in seeing that the 

 so-called pollen-grains are really spores, as shown by their 

 sizes, shapes, markings, colours, but especially by their 

 becoming free and germinating. Indeed they can be 

 made to germinate and develope in artificial nutritive 

 solutions containing sugar as readily as can the spores of 

 many Cryptogams. It is true we are accustomed to call 

 the outgrowth which the germinating pollen-grain emits 

 the pollen-tube, but this in no way vitiates the com- 

 parison, any more than in other cases where terms, 

 invented before the resemblances and differences were 

 understood, vitiate homologies elsewhere. The pollen- 

 grain being a spore, then, the rudimentary tissues which 

 it forms on germination are to be explained as some 

 structures developed by a germinating spore, and must 

 be compared accordingly. 



In the same way, having recognised the pollen-grain 

 to be a spore, we shall be prepared to see in the anther 

 a spore-case a sporangium and if we find that in other 

 instances the development of spores in a sporangium 

 is preceded by the formation of a sporogenous tissue, 

 tapetum, and so forth, our confidence in the accuracy 

 of the homology is strengthened. And this we find to 

 be the case. 



