DORSIVENTRAL SHOOTS. ANISOPHYLLY 99 



part in this differentiation. Frequently also the chief axis has assumed 

 from the beginning a dorsi ventral character as an adaptation, as for 

 example in the ivy, whose climbing or creeping shoots are distichous, 

 whilst the shoots ascending from the substratum and which bear the 

 flowers are radial and exhibit evidently the primary form. 



C. ANISOPHYLLY. 



By anisophylly we mean that leaves of a different size and of 

 different quality appear on the different sides of plagiotropous shoots ; 

 the leaves which stand upon the upper side are usually smaller than those 

 upon the under side, but the converse is also sometimes the case. 



Anisophylly is a phenomenon which is repeated in the most different 

 cycles of affinity and also within the same cycle of affinity, even within 

 the same genus, and it appears in different degrees. It will therefore be 

 instructive to follow out some examples taken from different groups. All 

 the examples have this in common, that the anisophylly occurs exclusively 

 upon plagiotropous shoots and that it is a character of adaptation which 

 has an evident relation to the direction of the shoot and especially to 

 its position with regard to light. This does not mean that light is the 

 determining factor in all the phenomena of anisophylly, for, as will be 

 shown in Chapter II of the Fifth Section, a relationship of configuration 

 does not require to be dependent upon the factor to which it is adapted. 

 The case of Selaginella, which will be mentioned below, shows that 

 anisophylly may appear in a plant in consequence of special circum- 

 stances, although the shoots are commonly isophyllous. 



HISTORICAL. The striking inequality in the leaves of many dicotyledonous plants 

 has been long known, and has given rise in many cases to specific nomenclature, 

 for example, Goldfussia anisophylla. The brothers Bravais have discussed this 

 plant, and Weddell has excellently dealt with the anisophylly of Elatostemma and 

 other species in his monograph of the Urticaceae. Herbert Spencer in 1865 first 

 directed attention to the anisophylly of lateral shoots in plants with decussate leaves, 

 as well as to the connexion of the anisophylly of higher plants with external factors, 

 especially with light. He says * : ' A kindred truth, having like implications, comes 

 into view when we observe the relative sizes of leaves on the same branch, where 

 their sizes differ. Fig. 205 represents a branch of a horse-chestnut, taken from 

 the lowermost fringe of the tree, where the light has been to a great extent 

 intercepted from all but the most protruded parts. Beyond the fact that the leaves 

 are bilaterally distributed on this drooping branch, instead of being distributed 

 symmetrically all round, as on one of the ascending shoots, we have here to note the 

 fact that there is unequal development on the upper and lower sides. Each of the 



1 Herbert Spencer, Principles of Biology, ii. p. 134. 

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