140 
ORGANOGRAPHY. 
BOOK I. 
this process see page 145. 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 being thus in a state of dilatation, it does 
not differ 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 : occasionally it becomes rigid and twisted, 
so that the plant can climb by it. In Combretum it hardens, 
curves backwards, 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. 
When the petiole grows upon the angles of the stem it is 
called by Link jo. synedrus ; when between them, p. cathedrus. 
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 loinyed. 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. c). 
They may always be distinguished from true leaves by the fol- 
lowing characters : — 1. 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 which 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 [Capreolus and Clamcula of the old 
