120 RELATIONSHIPS OF SYMMETRY 



are chiefly disposed to one side which we may call the outer side ; the 

 leaf-apices do not stand opposite the points of insertion of the leaves, as is 

 usual in shoots with distichous leaves, but the leaves are twisted on them- 

 selves through as much as 90. The dorsiventral character appears then 

 in the erect forms, in that each shoot has an outer side with much leaf- 

 surface, and an inner side with little leaf-surface (the latter is that turned 

 upwards in Fig. 70). As now the individual shoots turn their inner sides 

 to one another, there is developed a shoot-system in which the leafage 

 is directed outwards, whilst at the same time the individual leaves are 

 so placed that the shaded side is less developed than the unshaded. 

 I regard this as an instance of ' exotrophy.' 



It may be stated as a general rule, that species of Begonia which have 

 short internodes have long-stalked leaves, and the converse is also true, 

 at least in the large-leaved forms. A careful investigation of the biology 

 of the different species in their native habitat would be of the greatest 

 interest, because their conditions of life are so variable. Some are root- 

 climbers like the ivy, for example Begonia scandens, B. fagifolia, and 

 others ; others again, like B. Rex, have thick rhizomes growing on the 

 soil or rocks, whilst again others have orthotropous shoots. Leaving 

 aside the climbing forms we might compare the relationships of symmetry 

 in the species possessing elongated internodes (Fig. 70) with those 

 observable in compound leaves with asymmetric leaflets \ and the shoot- 

 axis of Begonia would then correspond to a petiole and the single leaves 

 to the leaflets. 



Asymmetric leaves are found also on orthotropous shoots in other 

 plants, and it is probable that there, just as in the species of Begonia, 

 we have to do with an arrangement for bringing the leaf-surfaces towards 

 the outer side. Thus in Achimenes, one of the Gesneriaceae, which 

 possesses two-membered or three-membered leaf-whorls, the asymmetry 

 of the leaves is as in other species of Gesneriaceae not always, but 

 occasionally, conspicuous, and the leaves have a sickle-like curvature, 

 like that shown in the Begonia represented in Fig. 70. In a garden 

 plant of Achimenes Haageana which I examined, the shoots developed 

 from tubers stood crowded together and were anisophyllous, inasmuch as 

 the leaflets turned inwards were smaller than those standing towards 

 the outside, and the effect of the sickle-like curvature of the leaf- laminae 

 was to bring the leaf-surfaces more towards the outside. 



Inequality in the sides of compound leaves is also met with, and 

 it is brought about by the unequal development in size of the leaflets. 

 This may go to such lengths that some of the leaflets on one side of 

 the leaf may be aborted. We may first note that the stipules in many 



Of this more will be said below. 



