LEAVES. 59 



to be the petiole, which at first assumes the common form, and 

 afterwards becomes a Phyllodium (as a leafclike expansion of 

 the petiole is called), and is succeeded by the tendril, and 

 finally, by the cohering of its edges, forms the pitcher, whose 

 lid is the lamina of the leaf. 



97. An unusual development of certain parts often produces 

 great variations from the regular form. The petiole is the part 

 of the leaf that experiences most frequent changes from this 

 cause. In the Dioncea muscipula, we find the petiole expanded 

 into a phyllodium, and terminated by the lamina, bearing much 

 less the appearance of this part of the organ than the petiole. 

 The Orange, also, has an expanded petiole, with the lamina 

 articulated to its extremity. We have described, by authors, 

 foreign plants exhibiting remarkable variations from any regu- 

 lar type, in some of which the petiole is developed apparently 

 at the expense of the lamina, which is either entirely wanting 

 or but imperfectly developed. The leaves of the Onion are sup- 

 posed to be a development of the petiole with no lamina, and 

 many of the leaves of the monocotyledons are supposed to be 

 of the same nature, as the Iris, Calamus, &c. The excessive 

 development of the cellular substance in leaves often causes 

 great diversity in appearance, as in the various species of the 

 Mesembryanthemums (Ice-plant). This cause operates to pro- 

 duce the singular forms observed in the different species of this 

 genus. In the Begonia, the parenchymous development on one 

 side of the midrib is much greater than on the other, thus pro- 

 ducing the oblique or one-sided leaf. 



98. A want of development and hardening of the parts often 

 produces deviations from the usual forms. The existence of 

 spines at the extremities of the lobes of the Holly is owing to 

 these causes, and in some radiated leaves the veins seem to be 

 converted into spines, and in a species of the Prosopis " one 

 half of the leaflets contracts into a spine, while the other half 

 remains leafy. But the most singular instance of this kind of 

 deviation occurs in a palm called the Desmoncus, in which the 

 upper leaflets of its pinnated leaves contract and curve into 

 scythe-shaped hooks, by which the Desmoncus climbs, while 

 the lower leaflets retain the usual appearance of leaves." 



99. The petiole is sometimes entirely wanting, but at others 

 excessively developed. In sessile leaves it is absent, but in the 

 Nymphcea odorata, a Water-lily, it is even six or eight feet 



What is u phyllodium? — 97. Explain the Dionsea : the leaves of the 

 Onion. What various forms by parenchymous development? — 98. What 

 from want of development in the Holly ? What in Desmoncus ? — 99. What 

 different states of the petiole ? How in sessile leaves ? How in the Water- 

 lily? 



