184 ORGANOGRAPHY. 



have now to describe generally the distinctive characters of 

 the leaves in those three great divisions. 



1. Leaves of Dicotyledonous Plants. — In these the vena- 

 tion is reticulated in consequence of the veins branching in 

 various directions and becoming united with each other, so as 

 to form an angular network {fig. 291). In some plants, as in 

 Rajuiticulus Lingua, gramineus, &c., the so called leaves have 

 parallel veins, and have been therefore considered by some 

 botanists as presenting exceptions to the ordinary reticulated 

 venation of Dicotyledons, but these as we have seen, are not 

 true leaves, but phyllodes or transformed petioles. 



The leaves of Dicotyledons are very commonly articulated to 

 the stem, often compound, and variously toothed or incised at 

 their margins. 



2. Leaves of Monocotyledonous Plants. — In these the 

 venation is commonly more or less parallel: either from base to 

 apex (^(7. 290, a); or presenting one large central vein from 

 which secondary veins are given off, which proceed in a parallel 

 direction to the mai'gin, as already described in the Banana, &c. 

 {fig. 290, b). The leaves of plants belonging to the Natural 

 orders, Smilaceai {fig. 364), Dioscoreacea), and Trilliaceoe, as 

 well as some in the order Aracea?, present exceptions to this 

 character, for in them the veins branch in various directions 

 and form a network, as in the leaves of Dicotyledons. Some of 

 these plants, as the Dioscoreacea?, Smilacere, and Trilliacea;, 

 have been therefore separated from the class of Monocotyledons 

 by Lindley, and placed in one by themselves, called Dictyogens, 

 from the Greek word, signifying a net. We have already seen 

 that such plants also present certain differences in their stems 

 from those of Monocotyledonous Plants gcnci'ally. 



In Monocotyledonous Plants the leaves are not articulated ; 

 their margins are usually entire or free from toothings and in- 

 cisions of every kind. They are also commonly simple. Their 

 leaves arc often sheathing at the base ; and seldom have stipules, 

 unless the ligule is to be considered as analogous to them. 



3. Leaves of Acotyledonous Plants. — In i)lants of this 

 class wliich have leaves with a true fibro-vascular system or 

 veins, tlicsc are arranged at first, either in a pinnate or pal- 

 mate manner, but the extremities are always bifurcated or 

 forked {fig. 299). The leaves of ferns are commonly called 



fi onds. 



Such leaves arc usually not articulated ; either sessile, or 

 stalked ; frequently toothed, or incised in various ways ; and 

 often highly compound. 



9. development of leaves. 



Nearly all that we know upon the Development of Leaves 

 is due to the admirable investigations of Dc Mcrcklin and 



