THALLOPHYTA. 641 



ber of zoospores. These zoospores, however, never become free like those of Pedi- 

 astrum, but remain joined together by strands of protoplasm, and after a certain 

 amount of shifting backwards and forwards, come to rest with their ends in contact. 

 Each then gradually assumes the characters of a Hydrodictyon-cell, the young 

 colony eventually escaping from the mother-cell (figs. 370 ^- *■ ^). Gametes are formed 

 in the same way as zoospores, but are smaller and more numerous. The spherical 

 zygote gradually increases in size, and its contents breaks up into 2-5 large zoo- 

 spores, which develop into large cells with pointed processes, the so-called poly- 

 hedra. In the interior of each polyhedron an embryonic Hydrodictyon-net is 

 developed from swarm-spores, and in the cells of this ordinary Hydrodictyon 

 colonies are found. 



It has been shown experimentally that any Hydrodictyon not above a certain 

 size and age is capable of producing either zoospores or gametes, and that the 

 stimulus to the formation of one or the other is given by external conditions. 

 Thus bright light, fresh water rich in inorganic nutritive salts, and fairly high 

 temperatures, are favourable to the production of zoospores, while the reverse of 

 these conditions, and especially the presence of organic substances, such as sugar, 

 tend to make the cells of a net produce gametes. The conditions favourable to- 

 zoospore -formation are also of course, favourable to active vegetative growth, 

 and no doubt the abundant formation of new protoplasm is a necessary preliminary 

 to the production of zoospores. A slight check to the processes of assimilation and 

 growth is apparently necessary in order to give play to the zoospore-forming forces. 

 Thus, experimentally, a change from a strong solution of nutritive salts to fresh 

 water will induce the formation of zoospores in nets which would simply have gone 

 on growing if left in the nutritive solution. A similar check is probably given by 

 the waning light in many Algae in which zoospores are produced at night. For the 

 production of gametes, on the other hand, an actual reversal of the conditions 

 favourable to growth is necessary. In nature this probably happens when by very 

 active growth the whole of the water of a pool is filled with nets, the inorganic 

 food and oxygen are exhausted, and the normal chemical processes of the cell 

 receive a check. The formation of gametes and zygotes under these conditions is 

 obviously adaptive, since the zygote can, although it need not, rest during several 

 months till the conditions are quite altered. We may therefore conclude that, 

 whereas zoospores are especially designed to multiply and distribute the species, 

 zygotes are intended to preserve it under unfavourable conditions. It is probable 

 that the production of large zoospores and polyhedra is a necessary part of the 

 life-cycle following the germination of the zygote, and cannot be altered by the 

 incidence of difierent conditions. 



Alliance VII. — SiphonesB. 



Thallus consisting of a tube, often much branched, and containing many nuclei. 



This tube is the production of a single cell, but in the more complicated forms is 



often shut ofF into compartments by transverse septa. Reproduction by zoospores 

 Vol. II. 91 



