30 



STRUCTURE OF LEAVES. 



and to the second we give the name of persistent ; the leaves of 

 fir trees are persistent. 



15. When all parts of the leaf are equally adherent to each 

 other, it is named a simple leaf, whatever may be the divisions 

 of its blade ; for example, the leaves of the lilac, the ranunculus, 

 of the vine, &c. (see figs. 17 to 57) ; sometimes the same tail 

 or peduncle supports several petioles, each of which is articu- 

 lated upon this peduncle, as it itself is upon the stem, and then 

 this assemblage is called a compound leaf. (Examples of com- 

 pound leaves are seen in the sensitive plant, the leaves of the 

 acacia, of the chestnut, &c. See Jigs. 58 to 74.) 



16. The fibres, by expanding 

 in the limb, constitute the nerves 

 of the leaf, and the cellular 

 tissue lodged between these 

 bundles of fibres, thus ramified, 

 constitutes the paren'chyma^ 

 of the leaf (fig. 15). 



17. The form of the leaf 

 depends principally upon the 

 disposition of the nerves ; in 

 general, the nerves expand on 

 a single plane so as to form a 

 plate or membrane with two 



Fig. 15. SECTION OF A LEAF. 



surfaces, a superior and an inferior; but they sometimes ramify 

 in all directions, and then give rise to leaves characterized by be- 

 ing thick, cylindrical, triangular, or swelled, as we observe in 

 certain fleshy plants. The large nerves that arise immediately 

 from the petiole are called primary nerves (figs. 25 and 26) ; 

 those which arise from the latter are secondary nerves (fig. 28) ; 

 we sometimes give the name of tertiary nerves (fig* 43) to those 

 ramifications which spring from the secondary nerves, and we 

 apply the name of veins of the leaf to those terminal divisions of 

 the nerves which are visible to the eye, but too small to make any 

 projection on the surface. [The veins are merely a continuation 



* PAREN'CHYMA (pronounced pareri-ke-ma). From the Greek, partg- 

 cliuein, to strain through. The spongy and cellular tissue of vegetables and 

 animals is so called. 



Explanation of Fig. 15. Section of a leaf showing the epidermis 

 (, a) ; the paren'chyma (&, &) ; the dense cellular tissue (c, c) ; and the 

 vessels (rf) of which its fibres are composed. 



35. What is a simple leaf? What is meant by a compound leaf? 



16. What are the nerves of a leaf ? What is paren'chyma ? 



17. Upon what does the form of the leaf depend ? What are primary 

 nerves ? What are secondary nerves ? What are the veins of leaves ? 



