FORMS OF LEAVES. 



strong towards the centre of the blade, like the leaves of the 

 foetid hellebore (fig. 72), and some of the arums, for example. 



21. The curvinerve leaves, in general, have a great number 

 of slightly projecting nerves, which most generally ramify near 

 their summit, and are often nearly parallel in the greater part 

 of their length (for example, the leaves of the narcissus and 

 fig,.fe.37). 



22. It sometimes happens that the space comprised betwixt the 

 nerves is not filled by cellular tissue, which produces a very 

 singular arrangement ; the leaf is then full of holes and resem- 

 bles a trellis-work (for example, the leaves of the Hydrogeton 

 fenestralis) ; or the holes are irregular, as we see in the leaves 

 of the Dracontium pertvsum. 



23. At other times the cellular tissue which surrounds the 

 nerves is spread out in such a way as to completely unite them 

 to their utmost extremity, in which case the leaf is said to be 

 entire (for example, the leaf of the lilac, and of the olive, figs. 

 22, 52, and 53). But between these two very different modes 

 of conformation, there is a great number of intermediate degrees. 

 Sometimes the paren'chyma completely unites all the ramifica- 

 tions of a secondary nerve, but does not extend between the 

 different nerves that arise from the primary nerve, so that the 

 blade is divided into several segments or lobes ; sometimes these 

 lobes are joined at the base or as far as the middle of their length, 

 and then the leaf is said to be partite or divided, and the intervals 

 between the lobes are called fissures (fig. 32). According to 

 the number of these fissures or divisions, the terms trifid, quin- 

 qvifid, &c., are used. In some instances this junction is com- 

 plete, but the paren'chyma which separates the last nerves does 

 not extend entirely to their extremity, and the edges of the leaf 

 are then dentate, as in the rose (fig. 47). When these small 

 marginal divisions are rounded instead of being pointed, they are 

 called crenulations, and the leaf is said to be crenulate (fig. 41). 



24. The two surfaces of the leaf are ordinarily covered with 

 an epidermis, which often has hairs upon the nerves, and stomata 

 on the paren'chyma ; these appendages and orifices are, in gen. 

 eral, especially numerous on the inferior surface; and on this 

 account it is almost always paler than the superior surface of the 



21. What is remarked of curvinerve leaves? 



22. Is the space between the nerves of the leaves always filled by cel r 

 lular tissue ? 



23. What is meant by an entire leaf ? When are leaves partite ? What 

 are fissures of leaves ? What is the difference between a dentate and a 

 crenulate leaf? (Dentate, from the Latin, dens, a tooth.) 



24. Why is the inferior surface of a leaf generally palest ? What is 

 found in leaves besides the nerves and cellular tissue ? 



