CHAP. II. LEAVES. 1 1 7 



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 cylindi'ical : 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 wingecL 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 {fig. 57.). 

 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 peculiai'ity 

 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 Clavicida of 

 the old botanists). It is one of the contrivances employed by 

 nature to enable plants to support themselves upon others that 



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