F UNGI. LICHENS. 



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this case the food-supplying Alga usually undergoes considerable change of habit, at 

 least in the outlines of its form, by the influence of the Fungus that preys upon it, just 

 as Euphorbia Cyparissias suffers from the aecidium that lives on it. The greater part 

 however of the Lichen-fungi employ as their hosts the Chroococcaceae and Palmella- 

 ceae which form coatings and cushions on moist soil, on the bark of trees and on stones. 

 The tissue of the Fungus grows so copiously around and among the cells and cell- 

 families of these Algae, that the latter at length appear to be merely dispersed through 

 the compact hyphal tissue, or to form a distinct layer, the gonidial layer, in it. 

 The Algae thus completely enclosed by their parasites are not impeded in vegeta- 

 tive growth or multiplication, though they are subject to other disturbances of their 

 development ; but if they are freed from the Fungi which are assailing them, they 

 proceed with their normal development, and they have been several times known to 

 form zoogonidia. 



We will first of all consider the Lichen as a whole, as it presents itself in nature, 

 where the Alga which supplies the nutriment appears under the name of gonidium as an 

 element in the construction of its thallus, and will afterwards examine further into the 

 algal nature of the gonidium. The thallus in the Lichens is often a crust overlaying 



FIG. 73. A piece of the foliaceous thallus of 

 Peltigera horizontalis ; a the apothecia, r the 

 rhizines. Natural size. 



FlG. 72. A and B Graphis elegans, a crustaceous 

 Lichen on the bark of Ilex aquifoltutn. A natural size. 

 B slightly magnified, C another crustaceous Lichen, Per- 

 tusaria Wulfeni. Slightly magnified. 



FIG. 74. A gelatinous Lichen, Collemt 

 pulposum. Slightly magnified. 



stone and bark, or insinuating itself between the laminae of the bark of woody 

 plants and sending out its fructifications only above the surface. These crustaceous 

 Lichens are so closely attached to the substratum, at least on their under surface, that 

 they cannot be removed from it entire and without injury to the thallus (Fig. 72, A, B, 

 C). These forms pass by intermediate steps into the foliaceous Lichens, in which the 

 leaf-like thallus forms flat, often crisped expansions, which can be removed in their 

 entirety from the substances on which they have grown, the soil, stones, moss or 

 bark being fastened to them only in places by special organs of attachment, named 

 rhizines. The foliaceous thallus not unfrequently attains considerable dimen- 

 sions, measuring sometimes in the large species of Sticta and Peltigera a foot in 

 diameter and a half to one millimetre in thickness, and then inclines to assume on the 

 whole a circular outline and has rounded indented lobes at the growing margin (Fig. 

 73, and Fig. 74). A third form of the Lichen-thallus, which is also connected by 

 intermediate forms with the previous ones, is seen in the fruticose Lichens, which are 



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