82 



STRUCTURE OF LEAVES. 



Callitriche, spiral vessels have been seen. In some instances there is 

 only a network of filamentous-like cells formed (fig. 146), the spaces 

 between which are not filled with parenchyma, giving a peculiar 

 skeleton appearance to the leaf, as in Ouvirandra fenestralis (lat- 

 tice plant). Such a leaf has been cal\e& fenestrate (fenestra, a window). 

 A leaf, whether aerial or submerged, generally consists of a flat 

 expanded portion (fig. 147 I), called the blade, limb, or lamina, of a 

 narrower portion called the petiole (petiolus, a little foot or stalk) or 

 stalk (fig. 147 p), and sometimes of a portion at the base of the 



petiole, which forms a sheath or vagina 

 ' ' (fig. 147?), or is developed in the form 

 of leaflets, called stipules (fig. 205). 

 The sheathing portion is sometimes in- 

 corporated with the stem, and has been 

 called tigellary (tige, Pr., a stem or 

 stalk) by Gaudichaud. These portions 

 are not always present. The sheath- 

 ing or stipulary portion is frequently 

 wanting, and occasionally only one of 

 the other two is developed. When 

 a leaf has a distinct stalk it is called 

 petiolate ; when it has none, it is sessile 

 I sit). When sessile leaves embrace the stem, 

 they are called amplexicaul {amplexor, I embrace, and caulis, a 

 stem). The part of the leaf next the petiole or the axis is the 

 base, while the opposite extremity is the apex. The surfaces of 

 the leaf are called the pagince (pagina, a flat page), and its edges 

 or margins form the circumscription of the leaf The leaf is usually 

 horizontal, so that the upper pagina is directed towards the heavens, 

 and the lower pagina towards the earth. In some cases leaves, or 

 leaf-like petioles, are placed vertically, as in Australian Acacias, 

 Eucalypti, etc. In other instances, as in Alstromeria, the leaf be- 

 comes twisted in its course, so that what is superior at one part 

 becomes inferior at another. 



The upper angle formed between the leaf and the stem is called 

 its axil (axilla, armpit), and everything arising at that point is called 

 axillary. It is there that leaf-buds (p. 108) are usually developed. 

 The leaf is sometimes articulated with the stem, and when it falls off 

 a scar or cicatricula remains ; at other times it is continuous with it, 

 and then decays gradually, while still attached to the axis. In their 

 early state all leaves are continuous with the stem, and it is only iu 

 their after growth that articulations are formed. When leaves fall 



Fig. 147. Leaf of Polygonum Hydropiper, with a portion of the stem bearing it. I, Limb, 

 lamina, or blade, p, Petiole or leaf-stalk, g, Sheath or vagina, embracing the stem, and 

 terminated by a fringe, / 



Fig. 147. 

 .s, from sedeo. 



