THE VASCULAR CRYPTOGAMS 23 



It may seem strange to us at first to find a cell 

 belonging to a plant swimming actively about, as if it 

 were an animal. When the first examples of such 

 moving vegetable cells were observed, more than fifty 

 years ago, the discoverer was so much astonished that 

 he thought he had caught the plant at the very moment 

 of its turning into an animal ! Now, we know better. 

 Actively moving cells are produced by most cryptogamic 

 plants ; sometimes they are male cells, as in Selaginella ; 

 sometimes they are sexless spores (see below, p. 151). 

 Movement, in fact, is not specially characteristic of 

 animals as distinguished from plants, for all protoplasm 

 is capable of spontaneous motion. We have seen this 

 already in the case of Elodea (Part I. p. 42), only there 

 the movements go on within a closed cell- wall. Wher- 

 ever movement is of advantage to the plant, we find that 

 its protoplasm can show itself just as active as that of 

 animal cells. In plants, however, owing to their different 

 mode of nutrition, the necessity for locomotion arises less 

 often. 



We cannot follow the fate of the spermatozoids any 

 further, until we have seen how the megaspores germinate. 

 Before we go on to this, however, we will carry the com- 

 parison between a microspore and a pollen-grain, rather 

 further than we have done hitherto. 



If we refer back to the account given in Part I. (p. 

 269) of the germination of the pollen-grain in the 

 Spruce Fir, we shall recall the fact that several cell- 

 divisions take place before the generative cells are formed. 

 In like manner we have found several cell-divisions in 

 the microspore of Selaginella before the spermatozoid 

 mother-cells are formed. 



The little prothallus-cell, which is first cut off, probr 



