CHAP. II. 
LEAVES. 
117 
ture of this process see page 122. In the Asparagus, 
the petiole has the form of a small sheath, is destitute of 
blade, and surrounds the base of certain small branches 
having the appearance of leaves ; such a petiole has been 
named hypophyllium by Link. In Trapa natans, Pontedera 
crassipes, and other plants, the petiole is excessively dilated 
by air, and acts as a bladder to float the leaves : except in 
this state of dilatation, it differs in no wise from common 
petioles : it has, nevertheless, received the name of vesicula 
from De Candolle, who considers it the same as the bladdery 
expansions of Fuci. The petiole is generally straight : occa- 
sionally it becomes rigid and twisted, so that the plant can 
climb by it. In Combretum it hardens, curves backward, 
loses its blade, and by degrees becomes an exceedingly hard, 
durable hook, by means of which that plant is able to raise 
itself upon the branches of the trees in its vicinity. 
It has been said that the figure of the petiole usually ap- 
proaches more or less closely to the cylindrical : this, however, 
is not always the case. In many plants, especially of an herba- 
ceous habit, it is very thin, with foliaceous margins ; it is then 
called tcinged. There are, moreover, certain leafless plants, 
as the greater number of species of Acacia, in which the 
petiole becomes so much developed as to assume the appear- 
ance of a leaf, all the functions of which it performs. Petioles 
of this nature have received the name of Phyllodia [Jig, 57.). 
They may always be distinguished from true leaves by the fol- 
lowing characters: — I. If observed when the plant is very 
young, they will be found to bear leaflets. 2. Both their sur- 
faces are alike. 3. They very generally present their margins 
to the earth and heavens, — not their surfaces. 4. They are 
always straight-veined ; and, as they only occur among dico- 
tyledonous plants vv^hich have reticulated leaves, this peculiarity 
alone will characterise them. 
But, besides the curious transformation undergone by the 
petiole when it becomes a phyllodium, there are several others 
still more remarkable ; among these the first to be noticed is 
the cirrhus or tendril ( Vrille, Fr. ; Capreolus and Clavicula of 
the old botanists). It is one of the contrivances employed by 
nature to enable plants to support themselves upon others that 
I 3 
