178 OEGANOGRAPHY. 



various directions and becoming united with each other, so as 

 to form an angular network {fig. 288). In some plants, as in 

 Eanunculus Lingua, R. gramineus, &c., the so-called blades 

 have parallel veins, and have been therefore considered by 

 some botanists as presenting exceptions to the ordinary reti- 

 culated venation of Dicotyledons, but these as we have seen (see 

 page 176), are not true blades, 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 Monocotyleuonous Plants. — In these the 

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

 apex {fig. 285, a); or the blade presents one large central vein 

 from which secondary veins are given off on each side, which 

 proceed in a parallel direction to the margins, as already 

 described in the Banana, {fig. 292). The leaves of plants 

 belonging to the Natural orders Smilacese {fig. 361), Dios- 

 coreacese, &c., as well as some in the order Aracese, 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 tliese plants, as the Dioscoreacese, 

 Smilacese, &c., 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. In 

 this work, as will be hereafter seen, in treating of the Clas- 

 sification of Plants, these plants are arranged as a sub-class of 

 the Monocotyledons. We have already seen (page 89), that 

 such plants also present certain differences in the structure 

 of their stems from those of other Monocotyledonous Plants. 



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 are often sheathing at the base ; and seldom have stipules, 

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



3. Leaves of Acotyxedonous Plants. — In plants of this 

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

 veins, these are arranged at first, either in a pinnate or palmate 

 manner, but their extremities are always bifurcated or forked 

 {fig. 293). The leaves of ferns are commonly cdiWedi fronds. 



Such leaves are usually not articulated ; either sessile, or 

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

 often highly compound. 



