190 



BOTANY 



sp 



B 



cells are distributed, either without any definite order (Hoomerous) 

 or in definite layers (Heteromerous). In most of the prostrate 

 forms the latter arrangement is the -rule. A section of one of these 

 (Fig. 153, B) shows the densely interwoven and often coherent 

 superficial hyphae, forming a tough outer rind or cortex, beneath 

 which is a somewhat looser stratum, in which the green cells form a 



continuous layer. The inner portion 

 of the thallus is made up of loosely 

 interwoven hyphae, forming a sort of 

 pith. The lower part of the thallus 

 is usually quite destitute of green 

 cells, and often develops rootlike 

 outgrowths, which fasten it to the 

 substratum. 



The fruticose Lichens, such as the 

 common Usnea barbata and the strik- 

 ing Ramalina reticulata (Fig. 156, A), 

 very common in the coast region of 

 California, are attached either by a 

 small disk, or in the latter sometimes 

 merely by being caught by the small 

 twigs upon which a fragment has 

 fallen. In these forms it is evident 

 that the substratum serves simply as 

 a point of attachment. 

 Where the gonidia of the Lichen are filamentous Algae, the growth 

 of the Lichen follows very closely the form of the Alga, which is 

 only slightly invested with the hyphae. 



Chemical Peculiarities of Lichens. The young hyphae usually show 

 the reaction of pure cellulose, but hater the cell-walls become modi- 

 fied, forming either Fungus-cellulose or a change into a gelatinous 

 substance, lichenin, or isolichenin, the latter substance turning blue 

 on the application of iodine, thus reacting like starch. A great 

 variety of peculiar products, such as special organic acids, pigments, 

 and various excretory products, are also found in the Lichens. 



a 



FIG. 154. A,Xanthoriaparietina, 

 filament from a germinating 

 spore attaching itself to cells of 

 Pleurococcus, a (X 950). (After 

 DE BAKY . ) B , Ramalina reticu- 

 lata, filament sending a haus- 

 torium into a Pleurococcus cell 

 (X900). (After PEIRCE.) 



Reproduction 



The thallus of a Lichen may multiply by means of fragments torn 

 off accidentally, or by the detachment of special bodies known as 

 "Soredia." These consist of roundish bodies composed of a tangle of 

 hyphae enclosing a number of the green cells. These soredia are some- 

 times formed in large numbers upon the surface or margins of the 

 thallus, where they form a greenish gray powder. Conidia, or non- 

 sexual spores like those of many ordinary Ascomycetes, occur in a 



