THE PAGE OF NATURE. 71 



ing a carbonaceous, beautifully-sculptured crust, and ele- 

 gant fructification. The foliage of the Thibetian wonder 

 may, therefore, be indebted for its singular markings to 

 a species of Limboria; and the characters on the bark 

 and branches may have been caused by an unknown 

 opegrapha. In fact, the counterpart of these inscrip- 

 tions has been discovered by Hooker and Thomson in 

 Khasya, on the leaves of a species of Symplocos. 



Let us glance at some of the peculiarities of the 

 lichens, and see if nature has not assigned them a higher 

 and more important commission in her great household, 

 than merely ornamenting old walls and ruins, and 

 covering trees with a shaggy mantle. 



Lichens, it has been said, are exceedingly simple in 

 their construction. They are composed of two parts, the 

 nutritive and the reproductive system. The nutritive 

 portion is called the thallus, which, in the typical plant, 

 spreads equally on all sides from the original point of 

 development, in the form of an increasing circle ; the 

 circumference of which is often healthy and vigorous 

 while the central parts are decayed or completely want- 

 ing. It is composed of two distinct tissues. The lower 

 or medullary portion is composed of spherical cells, rilled 

 with a green matter, which seem to be the active, vege- 

 tating part of the lichen. These cells frequently accu- 

 mulate in masses, burst through the layer above them, 

 and appear in the form of a green, tenacious powder on 

 the surface of the plant ; while they are capable, if de- 

 tached from the parent, of continuing the powers of cell- 

 development, and forming the nucleus of new lichens. 

 The external or cortical layer, on the other hand, is 



