<r> 



18 VEGETABLE ORGANOGRAPHY 



as if it were an incrustation. I think tbat an exudation 

 of tlie base of the Lichen dissolves a little of the stone, 

 and so combines v^ith it as to cause this kind of union : 

 it is also in the same manner, I presume, that certain 

 Lichens sink into calcareous stones, as they advance in 

 age, as, for example, Verrucaria rupestris, &c. 



The Thalli of Lichens are sometimes alike throughout 

 their whole surface. This happens when they are upright, 

 and equally exposed on all sides to the air and light ; they 

 then have most particularly the appearanceof a cylindrical 

 stem, as in XJsnea, Cladonia, Sec, or compressed, as in 

 Physcia^ and certain species of Rocella. At other times 

 the two surfaces are dissimilar. This happens in those 

 that are in the form of leaves or scales, and in a hori- 

 zontal situation ; the upper surface, being exposed to the 

 action of the air and light, is firmer, harder, and more 

 coloured, and acts, thus to speak, the part of bark ; the 

 lower is softer, more tender, and paler, and most fre- 

 quently bears hairs or cramps : it is by this surface also 

 that the absorption of water most usually takes place. 

 This fluid penetrates the whole of the Thallus, when a 

 part of it is plunged in, and in many cases its trace may 

 be followed by means of the absorption of coloured 

 liquids. 



One of the circumstances, which, according to 

 M. Fries, appears most powerfully to modify the vege- 

 tation of Lichens, is, that it is very frequently interrupted 

 by atmospheric changes, being suspended during drought, 

 and regaining its activity during damp seasons. 



It results from these frequent interruptions, that their 

 life may be much prolonged, and one frequently finds in 

 the same plant parts which appear dead, and others 

 which continue to vegetate. When the vegetation is 

 renewed under circumstances very different from those 

 in which the life of the individual commenced, it may 



