102 



THE LEAVES OP PLANTS. 





ligula. 



b, sheathing. 



c, ligula. 



length the whole is perfected it may be so closely connected with the stem that it does 

 not break off when the leaf has decayed, but hangs with the remains 

 of the leaf until the following season. A stem thus covered is said 

 to be induviate ; but in a majority of cases the petiole falls from the 

 stem, and leaves a mark which is known as the cicatricide. The 

 angle between the point of insertion of the petiole and the stem ia 

 termed the axile or axilla, and is the normal position of the leaf-bud 

 and the flower. 



Petioles have several important modifications. Thus in certain 

 so-called leafless plants, as the acacias, they assume the function of 

 leaves, and are termed phyllodes; but that they are veritable petioles 

 is proved by the fact that they bear leaflets at the earliest stage of 

 their development, and have parallel veins, although occurring in 

 of a grass, with exogenous plants. Such are the petioles in the Dioncea muscipula or 

 il fl ^petiole and Venus' s fly-trap (Fig. 1), in which plant they are expanded laterally, 

 and resemble the true leaves. This modification is due to an unu- 

 sual development laterally ; but there is another in which it proceeds 

 solely in the longitudinal direction. Such are tendrils, or spiral -spring- 

 looking organs, formed sometimes at 



the free ends of leaves, as in the pea, 



and at others at the side of the petiole 



itself, which twist around any fixed 



body to seek support for the climbing 



plant. (See Fig. 185.) 



There is yet a still more curioua 



modification of development, that in 



which the petiole enlarges, not only 



longitudinally and transversely but 



within itself, by the separation of its 



vessels and the increased deposition 



of connecting cellular substances. 



Thus the petiole becomes a tube, 



closed at the end by which it is at- 

 tached to the stem, and open at the 



other which is opposed to the blade of 



the leaf. This is the explanation of 



the formation of the interesting organs 



known as pitchers (Fig. 185*, p. 103) 



the pitchers themselves being the peti- 

 oles, and the moveablo lid which closes 



them being the true leaf. These 



pitchers have a further interest in 



the functions assigned to them of 



containing a watery fluid, and in the 



unique fact of the secretions of this 



fluid by certain glands formed within 



them at their base. In certain plants Fig * 185 The tendril > or elongated petiole or midrib. 



they are true fly-traps, and thus become direct organs of nutrition to the plant. 



