6o 



hardened ; these occur in the majority of the higher foliose and 

 fruticose lichens. 



i. In the lower crustaceous lichens the rhizoids are not dis- 

 tinguishable in structure from the hyphae of the thallus proper. 

 They form a network of branching filaments whose predominating 

 direction is downward into the substratum ; in the case of bark- 

 lichens this network surrounds the cork cells. Lindau and others 

 have proved conclusively that the hyphae never penetrate the intact 

 cells of the vegetable substratum. Among the hypophloeodal lichens 

 there are no rhizoids structurally recognizable as such. Since the 

 thallus is entirely enclosed by the substratum all the hyphae can 

 assist in absorbing soluble food substances ; the necessary me- 

 chanical support and protection is given by the tissue of the 

 substratum, so that the entire hyphal network is devoted to the ab- 

 sorption and carrying of food substances, which is the prime function 

 of rhizoids among the crustaceous lichens. In the lower lichens 

 saprophytism still predominates, indicating a close functional rela- 

 tion to fungi. 



Structurally the rhizoids of crustaceous rock-lichens are very 

 much like those of crustaceous bark-lichens. A prevailing mistaken 

 notion is that the rhizoids penetrate the solid rock ; this is mechani- 

 cally impossible ; the rock particles nearest the surface are, how- 

 ever, disintegrated by acid secretions from the rhizoids, whereupon the 

 hyphae are enabled to enter the openings and crevices thus formed 

 and enclose the loose insoluble rock particles ; the whole is held 

 together by the gelatinized cell-walls of the rhizoids. In this manner 

 the rhizoids may "penetrate" the rock to a considerable depth; in 

 fact, some lichens are entirely imbedded in the loose rock material. 

 The rhizoids of calcareous rock-lichens contain the oil-cells which 

 have been mentioned already. 



2. In some of the typically foliose thalli we find very long rhiz- 

 oids, consisting of single unbranched or sparingly branched hyphal 

 threads (Mallotium). The cell-walls are quite firm and considerably 

 thickened ; the walls also contain a coloring substance, usually 

 dark. Since these rhizoids are frequently aerial, that is having no 

 connection with the substratum, it is reasonable to suppose that they 

 serve to retain moisture ; they are also supposed to protect the 

 plant against the attacks of animals, especially snails (Zukal) ; in 

 other respects the^ resemble the forms described in the following 

 paragraph. -J 



