THE ALG.E 



151 



as it does so, to adapt itself to the width of the passage 

 (see Fig. 67, A). On first becoming free from the mother- 

 cell, the spore is enclosed within a delicate membrane 

 derived from the ectoplasm of the mother-cell, which 

 soon disappears, so that now the reproductive cell is 

 completely at liberty. In shape it resembles a pear, 

 the more pointed end being colourless ; the chloroplast 

 occupies the wider part, in which also the nucleus is 

 contained. There is no cell-wall, and the whole spore is 

 a purely protoplasmic structure. The cilia form a fringe 

 around the narrow end (see Fig. 

 67, B). Their oscillations set 

 the spore in motion, and now it 

 swims off through the water, 

 rotating on its axis, and advanc- 

 ing with the pointed end foremost. 



This is the first instance of 

 an actively-moving spore that we 

 have met with ; among the higher 

 Cryptogams already described it 

 is only the male cells or sperm - 

 atozoids which are capable of 

 locomotion ; in a large proportion of the Algae this power 

 extends also to the spores. On account of its active 

 movements such a spore as that of CEdogonium is called 

 a zoospore, for when first discovered these moving cells 

 were thought to be of animal nature. We now know 

 that spontaneous movement is a power common to all 

 protoplasm, whether belonging to a plant or an animal. 



The zoospore swims about for some time (an hour or 

 so) ; it is sensitive to light, swimming towards light of 

 moderate intensity, and retreating from it when too 

 bright. As the zoospore becomes older it avoids the 



FJG. 67. Zoospores of 

 CEdogonium ; A, escaping 

 from the mother-cell ; B, 

 free, with the fringe of cilia. 

 Magnified 350. (After 

 Pringsheim.) 



