ASYMMETRY OF LEAVES 119 



accurately upwards, we shall find upon consideration of the disposition 

 of the unequal sides of the leaves that there are two cases l : 



1. In thin-stemmed species, as for example Begonia scandens, B. 

 maculata, B. incarnata (Fig. 70), the larger sides of the leaves are 

 directed upwards as in Pellionia, and of the two stipules, which are of 

 unequal size, that which stands upon the illuminated side where it is 

 overlapped is the larger, whilst that upon the shaded side is the smaller. 



2. In other species, especially thick-stemmed ones like B. manicata 

 and B. Rex, the smaller sides of the leaves are directed upwards and 

 we have the converse of the former case ; but in the process of unfolding 

 of the bud a turning takes place by which the leaf-apices, and therefore 

 the smaller sides of the leaves, come to look downwards. In the erect or 

 obliquely ascending species the leaves stand in two rows with a divergence 

 of about 1 80, the lateral buds being in the axils of the leaves, but in 

 creeping stems the two rows of leaves approach one another upon the 

 upper side, as is shown in the diagram in Fig. 69, and the lateral 

 buds then stand upon the flanks, and there each of them appears in the 

 axil of one of the stipules an arrangement the advantage of which is 

 evident, and which is repeated in many dorsiventral shoots. 



The disposition of the plane of symmetry of the lateral shoots to 

 that of the chief axis varies. In many cases it falls at right angles to it, 

 and this which predominates in the thin-stemmed species appears to me to 

 be the ' typical ' case ; in other species the planes of symmetry intersect at 

 a less angle or may indeed coincide ' 2 . 



With regard to the biological significance of the oblique construction 

 of the leaves in Begonia, we may say in general with Herbert Spencer 3 

 that that side of the leaf is the smaller which is shaded, and that the 

 obliquity of the leaf is occasioned by its fitting itself to utilize the space 

 at its disposal ; this however does not explain ideologically the difference 

 in size of the stipules ; there is indeed accompanying the plagiotropous 

 growth of most species (some of which have again become orthotropous) 

 an inequality in the formation of the sides of the leaves which in many, but 

 not in all cases, appears to have a definite aim. 



Species of Begonia which have erect shoots possess, so far as my 

 observation goes, a bushy richly-branched habit. On examining a shoot 

 from above (see Fig. 70) we observe that the horizontal leaf-surfaces 



1 As the result of investigation of a number of species I can confirm what Sachs says in his text- 

 book, p. 209, regarding the relationships. Hofmeister's account is erroneous in more than one point. 



2 See Kolderup Rosenvinge, 1. c. on p. 70. Sachs and Eichler differ from Rosenvinge and do not 

 themselves agree. Sachs makes the angle of intersection in thick-stemmed species an acute angle, 

 Eichler in his diagram figures it as obtuse. According to Rosenvinge the plane of symmetry of 

 the lateral buds of B. hydrocotylifolia has the position assigned to it by Sachs, but in B. Rex it is 

 vertical from the beginning (see Fig. 69). 



3 Herbert Spencer, Principles of Biology, ii. p. 144. 



