STRUCTURE.] FOEMS OF PETIOLE. 297 



name of ligula (fig. 57. b.) ; for the nature of this process see 

 page 278. 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 hypophylium 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 

 generaUy 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^?. synedrus ; when between them, p. cathedrus. 



It has been said that the figure of the petiole usually 

 approaches more or less closely to the cylindrical : this, how- 

 ever, is not always the case. In many plants, especially of 

 an herbaceous habit, it is very thin, with foliaceous margins ; 

 it is then called winged. There are, moreover, certain leafless 

 plants, as some Woodsorrels (Oxalis) and the greater number 

 of species of Acacia, in which the petiole becomes so much 

 developed as to assume the appearance of a leaf, all the 

 functions of which it performs. Petioles of this nature have 

 received the name of Phyllodes (fig. 57. c). They may always 

 be distinguished from true leaves by the following characters : 

 1. If observed when the plant is very young, they will be 

 found to bear leaflets. 2. Both their surfaces 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 dicotyle- 

 donous plants which have reticulated leaves, this peculiarity 

 alone will characterise them. 



But, besides the curious transformation undergone by the 

 petiole when it becomes a phyllode, there are others still more 



