12. THE LEAF. 35 



degree of segmentation of the lamina depends upon the relation 

 between the growth of the various main axes and the marginal 

 growth of their respective wings (see Figs. 21 and 22). When 

 these keep pace with each other the lamina is_ altogether unseg- 

 mented, that is, its margin is entire : when the growth of the 

 axes is 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 seg- 

 ments, 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 inequalities 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 segmentation 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. 21) has an entire pinnate lamina; 

 the leaf of the Shepherd's Purse (Capsella Bursa-Pastoris, Fig. 23 C) is 

 simple, but the lamina is deeply pinnately lobed. Various forms of com- 

 pound pinnate leaves are represented by Fig. 19 C and by Fig. 23 B, Z>, 

 E, F, H, where the distinct segments or leaflets, termed pinna;, are inserted 

 on the common primary axis. In H each pinna is itself compound, being 

 segmented into pinnules which bear che 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 

 unijugate; 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. 23 D) when there is no terminal 

 leaflet, the leaf is paripinnate (Fig. 23 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. 29). 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- 



