DISPERSAL BY WATER. 803 



plasmic bodies (swarmspores) escape (c/. fig. 192, p. 17). What happens next differs 

 according to the species. In the genus Saprolegnia the individual swarmspores 

 have two cilia, by means of which they immediately swim away (see figs. 192 ^ and 

 192'); in Achlya, on the other hand, the swa-rmspores group themselves into a 

 round ball in front of the opening of the tube as they escape (cf. fig. 192 1- 2. s. *), and 

 at first possess no cilia. They surround themselves there with a delicate capsule, 

 which apparently consists of cellulose, but they do not remain long in this condition. 

 A few hours after, they leave the capsule and assume a bean-shaped form, being 

 now provided with cilia which enable them to swim about in the water. They only 

 swim about for a comparatively short time. When they have settled on some spot 

 they lose their cilia, surround themselves with a cell-wall, and become the starting- 

 point of a new plant; therefore they must certainly be regarded as ofishoots. The 

 Chytridiacese have a similar offshoot formation. These too are devoid of chlorophyll, 

 but they are true parasites, not saprophytes like the Saprolegniacese. They prefer 

 green water-plants for their hosts, penetrating into their cells, killing and destroying 

 the protoplasm, and then develop thick tubes which project beyond the host-plant, 

 and in which the protoplasm becomes divided up into numerous spherical portions. 

 The tubes open at their apex sometimes by the raising of an actual lid (see fig. 192^), 

 sometimes by the dissolution of a limited portion of the cell-wall, so that a hole 

 results from which the isolated protoplasts are expelled. On its escape each of 

 these offshoots is spherical or egg-shaped in form, and possesses a single long cilium. 

 This cilium serves as a swimming organ which in many species actually causes 

 a hopping and springing movement. In order to avoid repetition, we may refer to 

 the description of the swarmspore-formation given at vol. i. p. 29, in the case of the 

 Vaucherias and Sphaerellas. 



On the whole, as we have already stated, the formation of offshoots which swim 

 about independently in the water and seek out new spots suitable for settlement is 

 restricted to a very small section of water-plants. Offshoots which, after their 

 detachment from their place of origin, are carried passively by water currents with- 

 out exercising any directive influence, and are stranded at some distant spot, are of 

 much more frequent occurrence. Of these water-plants we might mention in the 

 first place the filamentous green Algse which cover with slimy masses the surface 

 of slowly moving water or stones at the bottom of rapid streams. In many of 

 these plants several times during the year do the dividing membranes between the 

 individual cells break down into mucilage so that the cells become free and are 

 ■carried away by the flowing water. Each of these cells may again give rise to a 

 new thread by repeated division. We cannot easily conceive a more simple method 

 of propagation and distribution than this. The offshoot-formation in the Florideae 

 is hardly less simple. Whether the whole plant is composed of rows or of open 

 networks of cells, four protoplasmic balls, the so-called tetraspores, are formed in 

 various situations on the plant; these are liberated into the surrounding water and 

 carried away by the current. They adhere to some firm spot under water and 

 there grow up into new plants. In most instances the protoplasm of the cell in 



