550 PHYSIOLOGY. 



that the movement of the juices in which chlorophyll-granules are con- 

 tained is directly dependent on the agency of light, especially of the more 

 highly refrangible rays of the spectrum. Under the influence of diffused 

 light the chlorophyll-granules range themselves parallel to the surface. 

 At night, as well as under the influence of direct light and of the most 

 luminous and least refrangible rays, they are disposed at right angles to 

 the surface, on the lateral walls of the cells. If, however, the light fall 

 from one side only, and the illumination be prolonged, the grains show 

 a tendency to accumulate on the side of the cell most brightly illuminated, 

 just as the zoospores of Algee do. Under unfavourable external condi- 

 tions (low temperature, age, or deficient light) the chlorophyll grains are 

 arranged against the sides of cells adjacent to others, and not on the free 

 surface as under normal circumstances. 



Analogous to the rotation of tlie protoplasm are the movements of 

 the ciliated zoospores of the Algae and of the ciliated spermatozoids 

 or antherozoids of the higher Cryptogamia and the Algae. 



Zoospores are formed by the contents of vegetative cells becoming 

 isolated from the cell-wall, and individualized into one (GEdogonium 

 fig. 505), a few (Ulva, Ulothrix* &c.), or numerous (Cladophora, 

 fig. 512, C, and Phseosporeoe) corpuscules, which break out from 

 the parent sac, and when free are seen to be provided with 

 vibratile cilia (2, 4, or many), and to swim about actively for a 

 period of from half an hour to several hours, then to settle down, 

 become encysted by a cellulose membrane, reassume the characters 

 of ordinary vegetative cells, and grow up into new plants by cell- 

 division. It has been observed that those zoospores with cilia at 

 one end direct that extremity (which is destitute of chlorophyll) 

 towards the light ; and, moreover, the locomotion of these bodies is 

 accompanied by a movement of rotation on their own axis. 



Spermatozoids are filiform bodies of various forms, mostly present- 

 ing one or more spiral curves, or minute globules, and usually 

 furnished with vibratile cilia. They are formed by a metamorphosis 

 of the protoplasmic matter of cells developed for the purpose in the 

 anthericlia of the Cryptogamia. They are extremely minute, but 

 move very actively when they escape from their parent cells, con- 

 tinuing to swim about for some time, being destined to find their 

 way to the archegoniuin (or to the spores in Algae), to perform the 

 fertilization of the germ-cell. Many, however, never reach this, 

 and they gradually dissolve away. 



In the Volvocineae (fig. 503, D) the separate primordial utricles lie 

 imbedded in a common envelope, without a membranous cell-coat, retain- 

 ing their vibratile cilia throughout life, only becoming encysted and 

 formed into proper vegetable cells when converted into resting-spores. 

 In the intimate affinity between these productions and the Protozoa, or 

 lower Infusorial Animalcules, we perceive the close bond which exists 



