100 ASCIDIA OE TITOHEES. 



leaves when there is a multiplication of their parts ; and a union of 

 two or more leaves, or of parts of leaves, occurs in many cases. 

 When two lobes at the base of a leaf are prolonged beyond the stem 

 and unite (fig, 171), the leaf is perfoliate (per, through, a,nd folium, 

 leaf), the stem appearing to pass through it, as in Bupleurum perfolia- 

 tum, and Chlora perfoliata ; when two leaves unite by their bases 

 they become connate (con, together, and natus, born), as in Lonicera 

 Caprifolium ; and when leaves adhere to the stem, forming a sort of 

 winged or leafy appendage, they are decurrent (decurro, to run down 

 or along), as in Thistles. 



The vascular bundles and cellular tissue are sometimes deve- 

 loped in such a way as to form a circle, with a hollow in the 

 centre, and thiis give rise to what are called fistular (fistula, a pipe) 

 or hollow leaves, and to ascidia (aexldiov, a small bag) or pitchers. 

 Hollow leaves are well seen in the Onion. Pitchers are formed either 

 by petioles or by laminse, and they are composed^ of one or more 

 leaves. In some Convallarias, two leaves unite to form a cavity. In 

 Sarracenia (fig. 203) and Heliamphora, the pitcher is composed 

 apparently of the petiole of the leaf. In Nepenthes (fig. 200) and 

 perhaps in Cephalotus, while the folding of a winged petiole, p, forms 

 the pitcher, a, the lid, e,, which is united by an articulation, corre- 

 sponds to the lamina. This kind of asoidium is called calyptrimor- 

 phous (xaXwr^a, a covering, and /ttofpij, form), and may be con- 

 sidered as formed by a leaf such as that of the Orange (fig. 201) ; 

 the lamina, e, being articulated to the petiole, p, which, when folded, 

 forms the pitcher. In Dischidia EaflSesiana, a climbing plant of 

 India, the pitchers, according to Griffith, are formed by the lamina of 

 the leaf, and have an open orifice into which the rootlets at the upper 

 part of the plant enter. These pitchers would seem therefore to 

 contain a supply of fluid for the nourishment of the upper branches of 

 the plant. In Utricularia, the leaves form sacs called ampullm. 

 Some suppose that pitchers are not due to folding and adhesion, but 

 that they are produced by a hollowing out of the extremity of the stalk. 



Structure and Form of Leaves in the Great Divisions of the 

 Vegetable Kingdom. 



Leaves op DiooTYiiBDioNS. — In Dicotyledons, the venation is 

 reticulated, the veins, coming off at various angles, form an angu- 

 lar network of vessels (fig. 151), and the tracheae communicate 

 with the medullary sheath. They are frequently articulated, ex- 

 hibit divisions at their margin, and become truly compound. There 

 are no doubt instances in which the veins proceed in a parallel man- 

 ner, but this will be found to occur chiefly in cases where the petiole 

 may be considered as occupying the place of the leaf. Examples of 



