NTRODUCTION. 



25 



Fig. 18. 



Leaves may be simple or compound. In the first, the petiole is undivided, 

 and the lamina consists of a single piece. A compound leaf consists of seve- 

 ral leaves or leaflets attached to a common petiole or rachis, quite distinct 

 and separate from each other. The forms of simple leaves are exceedingly 

 numerous and diversified, and should be attentively studied by the botanist, 

 as regards their contour, insertion, superficies and texture. A compound leaf 

 may be pinnate, or where a single series of leaflets arises from the petiole ; 

 bipinnate where a secondary series of petioles arises from it, also bearing a 

 second series of leaves; this division may go farther, and hence tripinnate, 

 &c. In the disposition of the leaflets 

 a leaf is said to be abruptly pinnate, 

 where the leaflets are even, and there 

 is no odd terminating one ; impari- 

 pinnate where this terminating leaf- 

 let is present. The number of leaflets 

 varies from very few to many, and 

 they may present all the diversities of 

 form exhibited in simple leaves. 

 Sometimes these forms are strangely 

 intermixed on the same plant, a 

 striking example of which often occurs 

 in the honey locust. 



According to their duration, leaves 

 may be caducous, when they fall 

 early ; deciduous when they are shed 

 before the appearance of the new leaf; 

 marcesce?it when they wither before 

 being detached ; and persistent when 

 they remain attached one or more seasons. In general, they are annually 

 produced in the spring, and fall off in the autumn. Plants which are desti- 

 tute of these organs are called aphyllous. As regards their position on the 

 stem, leaves are said to be radical when they are inserted into it, at or below 

 the surface of the ground ; and caidine when they arise from the stem at a 

 higher point. 



The internal structure of a leaf consists of two sets of veins or vessels, one 

 belonging to the upper surface, and conveying the sap of the plant to the ex- 

 pansion ; the other belonging to the under surface, and returning the elabo- 

 rated juices to the bark. These 

 ramify through a cellular tissue 

 called parenchyma, and con- 

 tainingnumerous green globules 

 termed chromule or chlorophyl- 

 lin, to which the colour of the 

 leaf is owing. These globules 

 are differently arranged in the 

 two strata of vessels; in the 

 upper one there are usually two 

 or more layers of compact 

 oblon 



long diameter perpendicular to 

 the surface; in the lower surface, these vesicles are very loosely arranged? 

 having numerous vacant spaces between them, communicating with the ex- 

 ternal air by means of stomata. In aquatic plants whose leaves float on the 



Leaf of Gleditsia. 



Fig. 19. 



Section of lily leaf. a. Cells of cuticle of upper surface. 

 o. Cells of cuticle of lower surface, c. Stomata. d. Paren- 

 veSlCleS, having their chyma of upper surface. e,f. Parenchyma of lower surface. 



