52 . PART I. THE MOEPHOLOGY OF PLANTS. [ 12. 



rather more vigorous than that of the corresponding wings, the 

 margin becomes somewhat uneven (dentate, serrate) ; when the 

 difference between them is considerable, the lamina is lobed; and 

 when still greater, it consists of a number of distinct segments, 

 leaflets, connected only by their common attachment to the mid- 

 rib, in the case of pinnate leaves, or to the petiole in the case of 

 palmate or peltate leaves. Whilst inequalties of the margin are 

 indications of branching, the lamina is regarded as simple so long 

 as the segmentation is incomplete; it is only when the segmenta- 

 tion is complete, as in the last-mentioned case, that the leaf is 

 said to be compound. 



The following examples will serve to illustrate the foregoing principles. The 

 simple leaf of the Beech (Fig. 30) has an entire pinnate lamina ; the leaf of the 

 Shepherd's Purse (Capsella Bursa-Pastoris, Fig. 32 C) is simple, but the lamina 

 is deeply pinnately lobed. Various forms of compound pinnate leaves are 

 represented by Fig. 28 C, and by Fig. 32 B, D, E, F, H, where the distinct 

 segments or leaflets, termed pinna, are inserted on the common primary axis 

 (phyllopodium). In H each pinna is itself compound, being segmented into 

 pinnules which bear the same relation to the secondary axis of each pinna as 

 that secondary axis does to the primary axis of the whole leaf ; such a leaf is 

 said to be bipinnate, and when the segmentation is carried further the leaf 

 becomes tripinnate, etc. 



In compound pinnate leaves, the leaflets are commonly opposite to each 

 other. When only one pair of leaflets is present, the leaf is said to be uniju- 

 gate\ when two pairs, bijugate-, when many pairs, multijugate. When the 

 axis (whether primary or secondary) is terminated by a leaflet, the leaf is said 

 to be imparipinnate (Fig. 32 D) ; when there is no terminal leaflet, the leaf is 

 paripinnate (Fig. 32 E). When, as in the Potato and Potentilla anserina, pairs 

 of small leaflets alternate with pairs of larger ones, the compound leaf is said 

 to be interruptedly pinnate. The difference in size of the leaflets is simply 

 due to the more active growth of the larger ones. 



The order of development of the leaflets of compound pinnate 

 leaves depends upon the position of the growing-point in the 

 longitudinal axis (see p. 17). When it is apical, the leaflets are 

 developed in acropetal succession (e.g. Pea, Ailanthus, etc.); 

 when it is basal, in basipetal succession (e.g. Myriophyllum, Poten- 

 tilla anserina) ; when there is both an apical and a basal growing- 

 point, in divergent succession, that is, both acropetally and basi- 

 petally (e.g. Achillea Millefolium, etc.). 



With regard to palmate leaves, Fig. 32 A, is an example of a deeply lobed 

 lamina; and B, of a compound palmate leaf. It will be observed that in the 

 compound palmate lamina of the Clover (Fig. 32, B) there are three leaflets; 



