338 



PLANT LIFE. 



plants may be surrounded liy a mycelium. The enslaved 

 green plants are generally unicellular or filamentous algK. If 

 the latter are the species whose colonies produce voluminous 

 gelatine, the texture of the lichen bod)' is gelatinous; other- 

 wise it is tough and leathery. Some 

 of the fungi which ordinarily associ- 

 ate themsehes with alga; to form 

 lichens may exist free as sapro- 

 phytes. The alga itself influences 

 the form of tlie thallus more or less 

 profoundly according to its relative 

 amount. The same fungus associ- 

 ated with different alga; produces 

 lichens which are described as dif- 

 ferent species, or even as different 

 genera. 



463. 2. Animals and algse. — 

 Melotism exists between animals 

 and alg;e. A'arious siuiple animals, 

 such as radiolaria stentors, hvdras, 

 sponges, echinoticrms, and worms, 

 enclose algK in their bodies and 

 utilize the products of their food 

 manufacture. The alga; thus ens]a\cd are all minute uni- 

 cellular forms which multiply within the animal body by 

 division (fig. 37S). 



Fig. 37,S. — .-V radinlarian ^Litlio- 

 ceTciis all 'luiaT is^, one ot the 

 microscopic single-celled animals 

 with a siliceous skeleton, .s, 

 formed by the outer portions of 

 the protoplasm, A', which is sep- 

 arated from tlie internal proto- 

 plasm, 7, by a perforated cap- 

 sule, t" : nu, nucleus ; f'xj, 

 threadlike protrusions of the 

 protoplasm. Embedded m the 

 outer protoplasm, A', are numer- 

 ous "yellow cells," 2, each with 

 its own cell-wall, nucleus, and 

 cliloroplasts. These are an alga, 

 called Zooxaiitlulhi niiiricoid. 

 Highly magnified. — After 

 BUtschli. 



C. Parasitism. 



464. I. Fungi. — .\. %crv large number of colorless plants 

 have ada]")ted themselves to li\'c upon li\'ing 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 l)oth are more or less seriouslv inter- 

 fered with, so as to jModiice disease, slight or gra\e, local or 



