NUTRITIVE ORGANS OF CELLULAR PLANTS. 317 



form of cylindrical stems, more or less branched, some- 

 times even being furnished with small flat expansions, of 

 a foliaceous appearance. Lastly, all these different 

 forms are, in several species, reduced to dimensions so 

 small that the whole of the plant (with the exception of 

 the organs of fructification) is only formed of a crust, 

 composed either of foliaceous scales, as in Squammaria 

 and Patellaria, or of small stems very close together, as 

 in Isidium, or of a granulated or pulverulent matter, 

 which may be considered as a mass of indistinct scales, 

 or imperceptible trunks, as takes place in Lepra and 

 Coniocarpon. In the midst of all these variations, the 

 internal substance only presents a mass of cellular tissue, 

 with round or elongated cellules, usually very small, and 

 close to one another. The most remarkable physiological 

 character of this tissue is, that when it is torn, the in- 

 ternal substance becomes green on exposure to the air 

 or light. This phenomenon is especially very remarkable 

 in the species which adhere to rocks. 



In general, the tissue of Lichens is formed of elon- 

 gated cellules in the parts which resemble stems or 

 branches, and of round ones in those which are like 

 foliaceous expansions. 



The surface presents remarkable differences : some- 

 times it is entirely smooth, at others it is provided with 

 hairs or ciliae of various kinds ; at times it is prolonged 

 into cramps, or roots, which serve to fix the plant, and 

 perhaps to absorb nutriment. It is especially in the 

 genus Psora, that these appendages appear to be true 

 roots implanted into the soil, and serve at the same time 

 to fix and nourish the plant. In most of the species 

 which grow upon rocks, or trees, there are neither 

 cramps nor roots, and the plant is fastened on the surface 

 which bears it, by means of a little disc at its base ; this 

 adheres to rocks in a very intimate manner, and seems 



