106 FIRST FORMS OF VEGETATION. 
higher cryptogams, and of the bulbils, stolons, etc., 
of the flowering plants. The green matter of the 
cells or endochrome is resolvéd into zoospores as 
in the conferve or fresh water alge, and in the 
stalked spores and reproductive cells of fungi,—a 
circumstance which brings lichens into close re- 
lationship in an important point with alge and 
fungi. The external or cortical layer called the 
hypha, on the other hand, is supposed by some 
botanists to serve the same purpose in the eco- 
nomy of the lichen as the bark does in that of 
the tree, viz.,as a protection to the lower, living 
layer, of the dead cellules of which it actually 
consists. In some species this outer covering is 
smooth, and in others covered with small hollows 
or pits, and sprinkled over with powdery warts. 
The lower surface of the lichen is usually of a 
paler colour than the upper, and is covered with 
hair or fibres which serve to fix the plant. 
A curious theory has recently been promulgated 
by Continental botanists regarding the parasitic 
nature of all lichens. They do not form a dis- 
tinct order of vegetation as we have been in the 
habit of regarding them, but are supposed to be 
produced by a combination of fungi and those 
confervoid algz which are universally distributed 
on bark, wood, rocks, and mosses, attaining their 
greatest development in moist and shady places. 
