ASYMMETRY OF LEAVES 125 



is symmetric whilst the other is asymmetric. The leaf is bipinnate ; 

 the terminal lobe is aborted and the lateral leaflets are of very unequal 

 size, the larger being strongly asymmetric, the smaller nearly symmetric. 

 The relationships of symmetry find an easy teleological explanation 

 here; if in the asymmetric leaflets the smaller side was as large as 

 the larger, overlapping would take place ; the smaller and inner leaflets 

 have only to fill up the interval between the larger. 



We are naturally led from a consideration of plants with asymmetric 

 leaflets to those in which the stipules are asymmetric. This is the case 

 in many Leguminosae, for example species of Vicia, Pisum (Fig. 75), 

 Orobus, and Lathyrus, and also in Rosaceae, for example in Spiraea 

 Ulmaria, Agrimonia, and others. We know nothing of the factors which 

 cause the asymmetry here, and can only bring it into relationship on 

 the one side with the position of the stipules, and on the other with 

 their function. It is usually that portion of the stipule which is turned 

 aivay from the leaf, in which the stronger growth is observed, and thus 

 the protection of the bud is favoured. Lathyrus Aphaca is interesting 

 in this respect (see Figs. 76 and 77), inasmuch as the stipules of the 

 fully-formed leaves are asymmetric whilst those of the arrested or the 

 tendrillous leaves are symmetric. The unilateral increase in the con- 

 figuration of the stipules will be understood in its teleological relation 

 by a glance at Fig. 78, which shows a transverse section through the 

 bud of Vicia Cracca. The leaves here are in two rows, as in other 

 Leguminosae, and are folded from the midrib, and each leaflet is similarly 

 folded. As the leaves are very hairy upon the outer side the bud is 

 well protected. Above and below however there are two longitudinal 

 strips which are covered by the stipules, st r st^ and it is clear that the 

 protection they afford to the bud will be all the better the more they 

 are extended on the sides which are directed away from the leaves, because 

 places occur there which are not protected by the leaves themselves. 

 This unilateral growth takes place, if some observations on Lathyrus 

 sativus enable us to judge, only relatively late, the stipules are primarily 

 laid down as symmetric structures. When the leaf itself, as in Lathyrus 

 Aphaca (Fig. 77), takes practically no share in the protection of the bud, 

 the stipules must take on the whole of this duty, and we can at least 

 teleologically understand that asymmetry is a consequence ; and the fact 

 that its development only takes place at a relatively late period gives 

 support to the assumption that the asymmetry is only a secondary 

 character of adaptation. 



