150 ABSENCE OF LEAVES IN FUNGI, PETIOLE; LEAFLETS. 



224. The case is different, however, in regard to the FUNGI. 

 These plants derive their nourishment from matter, which has 

 already been in a state of organisation ; and the condition in 

 which they receive it is such, that it does not require to be ela- 

 borated by exposure to the atmosphere, as it does in all other but 

 the parasitic plants already mentioned, which much resemble the 

 Fungi in habit. Accordingly the Fungi, the whole energy of 

 whose vegetation seems to be concentrated upon the propagation 

 of the race, do not possess anything analogous to leaves; and 

 seldom exhibit even such an expanded surface, as may be consi- 

 dered to replace them. It is very rare, too, that this surface is 

 green ; and, as will be hereafter shown (. 286), this green 

 colour in leaves is due to certain changes, which, from the con- 

 dition of the growth just mentioned, the Fungi do not need to 

 perform. 



225. We now return to the leaves of Flowering plants; 

 and shall trace in more detail their regular structure, the chief 

 varieties of this, and the functions which they are destined to 

 perform. And in the first place we shall consider their external 

 aspect. 



226. The leaf is usually borne upon a petiole or foot-stalk; 

 which connects it with the stem ; and it is at the bottom of the 

 petiole, that the separation from the stem takes place, when the 

 leaf falls off. By this it may be known what is really a single 

 leaf, and what is a collection of separate leaves. Not unfre- 

 quently a leaf is very compound in its structure, consisting of a 

 number of distinct leaflets, which might be regarded as so many 

 leaves. But if these leaflets all proceed from one foot-stalk, and 

 this drops off altogether at the accustomed period, they are to be 

 considered as only the subordinate parts of a single leaf. Many 

 such instances might be enumerated ; but it will suffice at present 

 to refer to the Ferns (.23), in which what appears to be the 

 stem is really but a leaf-stalk ; and what seem to be leaves 

 are only leaflets proceeding from it, and forming part of one 

 large leaf. 



227. There is not always such a definite distinction between 

 the flat expanded blade and the round and slender petiole, as, 



