338 



PLANT LIFE. 



plants may be surrounded by a mycelium. The enslaved 

 green plants are generally unicellular or filamentous algae. If 

 the latter are the species whose colonies produce voluminous 

 gelatine, the texture of the lichen body is gelatinous ; other- 

 wise it is tough and leathery. Some 

 of the fungi which ordinarily associ- 

 ate themselves with algae to form 

 lichens may exist free as sapro- 

 phytes. The alga itself influences 

 the form of the thallus more or less 

 profoundly according to its relative 

 amount. The same fungus associ- 

 ated with different algae produces 

 lichens which are described as dif- 

 ferent species, or even as different 

 genera. 



463. 2. Animals and algae. — 

 Helotism exists between animals 

 and algae. Various simple animals, 

 such as radiolaria stentors, hydras, 

 sponges, echinoderms, and worms, 

 enclose algae in their bodies and 

 utilize the products of their food, 

 The algae thus enslaved are all minute uni- 



Fig. 378. — A radiolarian (Litho- 

 cercus annularis), one of the 

 microscopic single-celled animals 

 with a siliceous skeleton, S. 

 formed by the outer portions of 

 the protoplasm, E, which is sep- 

 arated from the internal proto- 

 plasm, /, by a perforated cap- 

 sule, c ; nu, nucleus; /W, 

 threadlike protrusions of the 

 protoplasm. Embedded in the 

 outer protoplasm, E, are numer- 

 ous "yellow cells," z, each with 

 its own cell-wall, nucleus, and 

 chloroplasts. These are an alga, 

 called Zooxanthella nutricola. 



_ ly 



Biitschli. 



manufacture. 



cellular forms which multiply within the animal body by 



division (fig. 378). 



C. Parasitism. 



464. 1. Fungi. — A very large number of colorless plants 

 have adapted themselves to live upon living plants or ani- 

 mals which they force to act as their unwilling hosts. By 

 the presence of the parasite the normal functions of the host 

 or its normal growth or both are more or less seriously inter- 

 fered with, so as to produce disease, slight or grave, local or 



