100 THE LEAVES OF PLANTS. 



size, both longitudinally and transversely, than simple leaves. They are subject to 

 great variety of forma ; and in their development are guided by similar laws to those 

 already explained in respect of simple leaves. 



The most common form of compound leaf is the pinnate (Fig. 180), in which there 

 are a series of small leaves arranged on each side of the midrib. When they are in 

 pairs on opposite sides of the midrib, they are said to be opposite; and when single 

 they are termed alternate. In many instances the leaf is terminated by an odd leaflet 

 (Fig. 177), and the branch is said to be determinate; when otherwise, the development 

 of the leaflet has been arrested ; and if no flower exist at the end of the branch, it is 

 called indeterminate. An intermediate condition is found in such leaves as have small 

 foliaceous organs attached to the midrib between the leaflets ; and then the leaf is 

 termed interruptedly pinnate (Fig. 178). It is understood that the normal arrangement 

 of the leaflets is alternate, as may be inferred from a consideration of Fig. 150 ; for it 

 is there seen that, although each side is symmetrical, the primary veins (which would 

 form the midribs of the leaflets of a compound leaf) do not leave the midrib at points 

 directly opposite to each other. This is also deduced from the observation, that at the 

 formation of the first leaf at the first node (see page 58), there is no opposite leaf, but 

 that one is subsequently formed at the next node ; and hence it is inferred that when- 

 ever leaves are placed opposite to each other, as seems to be the rule in the development 

 of the leaflets of a compound pinnate leaf, there has been the suppression of an inter- 

 vening leaf and node. This suppression is carried to a yet greater extent in the arrange- 

 ment of leaves in whorls (Fig. 179) ; for then not only are there two opposite leaves, 



Fig. 179. Fig. 180. 



Fig. 179. A whorl of leaves surrounding the stem. 



Fig. 180. Representing at a the pinnate, and at b the hipinnate arrangement of leaves. 



but the number is increased to four, six, or more. In such instances there has been 

 an absence of as many nodes as there are leaves, except one. "We may also explain the 

 formation of leaves in whorls on the same principles that we have applied to pinnate 

 leaves, viz., that they are all the produce of a divided simple orbicular leaf, as in Fig. 

 154, in which each leaf incloses one primary vein, whilst a pinnate leaf is in like 

 manner the product of the division of such a leaf as delineated in Figs. 157 and 158. 

 This arrangement of leaves into alternate, opposite, and whorled, is also applicable to 

 leaves, of whatever kind, arranged around the whole branch or tree. In many instances, 

 and especially in the Umbellifera, the pinnae of the pinnate leaf are themselves sub- 

 divided, and then the leaf is termed bipinnate (Fig. 180), and is analogous to the 

 doubly-pinnatifid leaf in Fig. 165. 



