THE LEAVES OF VASCULAR PLANTS. 239 



bundle of the fibres of the leaf. Most frequently a leaf 

 is composed of a limb and petiole ; sometimes it is devoid 

 of one or the other : in these different cases, the petiole 

 assumes various appearances, and it is often difficult to 

 recognise its existence among the many forms it takes. 

 In order to follow it in all its metamorphoses, we will 

 first study it in leaves with ramifying nerves, where its 

 structure is more appreciable, and afterwards in those 

 with simple ones. 



It is among leaves with ramifying nerves, or those of 

 Dicotyledons, that the peculiar structure of the petiole is 

 most evident, and in which it can be studied under its 

 most simple form ; it is there presented under that of an 

 elongated and nearly cylindrical bundle of fibres; its length 

 is very variable — sometimes it is longer than the limb, 

 sometimes so short that it may be said not to exist. Its 

 form is either entirely cylindrical, — or slightly depressed, 

 — or hollowed into a channel, the upper side being plane 

 or concave, and the lower raised up into a dorsal angle, — 

 or, lastly, much compressed, as is seen in the Poplar, in 

 which this form of the petiole causes the extreme mobility 

 of the leaves. In ail these cases the petiole is simple, 

 composed of a certain number of fibres close to one 

 another, intermixed with elongated cellular tissue ; it 

 never bears stomata, but, like the nerves of which it is 

 the base, it is often furnished with hairs or glands ; its 

 colour is usually pale, its texture firm ; it does not 

 decompose carbonic acid, and assists but slightly, if at 

 all, in the evaporation of water. 



This ordinary form of the petiole is modified by 

 difierent cii'cumstances. 



1st. When the edge of the petiole is flattened, it 

 sometimes happens that it spreads out laterally into a 

 flat and foliaceous portion, perfectly resembling the 

 parenchyma of the limb. It is said then that the 



