io8 RELATIONSHIPS OF SYMMETRY 



lower forms. The leaves of one side, the upper, may be distinguished 

 only by their smaller size from those of the other side, the under, but 

 in extreme cases of anisophylly which are met with in many Urticaceae 

 for instance, Elatostemma, Pellionia, and others, there is not only the 

 difference in size but also a difference in structure and in function between 

 the leaves of the two sides. We may distinguish, although not sharply, 

 two cases in the one, only the lateral shoots of radial isophyllous chief 

 shoots are anisophyllous, and we may designate this lateral anisophylly ; 

 in the other, the whole shoot-system is anisophyllous and this may be 

 called habitual anisophylly. 



a. LATERAL ANISOPHYLLY. 



This is seen in most striking form upon the lateral branches of 

 trees and shrubs with large decussating leaves, for example, species of 

 Aesculus, Sambucus nigra, Acer campestre, A. platanoides, A. pseudo- 

 platanus and other species of Acer, species of Fraxinus, Staphylea pinnata ; 

 it occurs also in Catalpa syringaefolia which has three-leaved whorls. 

 The differences are less striking where the phyllotaxy is spiral, as for 

 example in Quercus referred to above (page 93), and in Abies which 

 will be mentioned below (page 250). 



This anisophylly is not however limited to trees and shrubs but is 

 found in herbaceous plants, for example in Urtica, species of Scrophu- 

 laria, and others ; tropical plants in particular furnish a large number of 

 examples. It is commonly absent from those of our shrubs which, as has 

 been mentioned above, have the leaves of their lateral shoots displaced 

 by torsion into one plane, and if it does occur upon them it is only in 

 the last leaves of an annual shoot which retain their primary position. 



We find not infrequently in species of ash and maple that the 

 upper leaf of the pair next to the terminal bud is developed as a bud- 

 scale, whilst the under one of the pair is a foliage-leaf, and this shows 

 that the formation of leaves upon the under side is furthered. In plants 

 with decussating leaves in which the leaf-pairs are not brought into one 

 plane by internodal torsion the leaves either remain in their normal 

 position, that is to say, two rows of leaves are lateral, one row is above, 

 and one row is below, or a torsion takes place by which the four rows 

 of leaves are brought into a diagonal position. In the first instance, 

 which is the common one in lateral anisophylly, the opposite leaves in 

 each leaf-pair of the lateral rows are of equal size ; in the second case, 

 which always happens in habitual anisophylly, the leaves of the two 

 rows towards the upper side are smaller than those of the two rows 

 towards the under side. In lateral anisophylly however a torsion may 

 also take place, as for example in the horse-chestnut. The larger leaves 



