n6 



RELATIONSHIPS OF SYMMETRY 



and of the configuration 



FlG. 67. Geranium pratense. Trans- 

 verse section through a seed. To the 

 right the hypocotyl ; to the left the two 

 cotyledons. The midrib of the cotyledon 

 is evident and it is clear that the in- 

 equality of the sides of the cotyledons 

 bears a relation to the available room 

 \vithin the seed-coat. 



of the seed ; it is not a phenomenon of 

 adaptation. In Fig. 67 we have a repre- 

 sentation of a cross-section of a seed of 

 Geranium pratense. The two cotyledons are 

 so folded that the smaller side of one is 

 invested by the larger side of the other, 

 and the arrest of the one side of each 

 cotyledon is evidently caused by its having 

 less room available than the other 1 . More 

 will be said about this in the section dealing 

 with the development of seeds. 



2. FOLIAGE LEAVES. 



We have to consider here both the asymmetry of the entire leaf and 

 the asymmetry of lateral pinnules. 



a. ASYMMETRY OF ENTIRE LEAYES. 



To this category belong, in the first place, the leaves of a number 

 of plagiotropous shoots, for example those of many grasses, of Colum- 

 nea Schiedeana and C. Kalbreyeri, of the juvenile form of Ficus 

 stipularis, of the Urticaceae mentioned on page 109 (see particularly 

 Fig. 62, representing Pellionia Daveauana), and of Centradenia ; also 

 the leaves of a series of shoots which are not plagiotropous, for example 

 the pendant sickle-like leaves of Eucalyptus globulus, and the leaves 

 of Rochea falcata 2 , which differ from most others in that the like-formed 

 sides of the leaves in each pair fall upon opposite sides of the median 

 of the pair (Fig. 68) in the terminology of Wydler the leaves are 'anti- 

 tropic '-whereas the similar sides of the leaves in a pair, whether they 

 be the small or the large, usually fall upon the same side. 



We cannot doubt that asymmetry of the leaves chiefly appears when 

 'their parts are unsymmetrically related to the environment 3 .' The 

 median plane of such leaves is usually oblique to the horizon to use 

 Wiesner's terminology they are ' klinotropic ' and the asymmetry stands 

 in connexion with the obliquity of this plane. It is not easy however to 

 determine the factors which condition this, and I must refer to a few 

 examples which may give us a starting-point for the solution of the 

 question. 



' The asymmetry in. the leaves of many species of Musa may be explained in the same way. 



2 The leaf-pairs of Rochea falcata cross each other at an oblique angle, and this is seen in even 

 higher degree in Mesembryanthemum. The biological significance of these relationships of con- 

 figuration in Rochea is still unknown. 



s Herbert Spencer, Principles of Biology, ii. p. 143. 



