xi] THE MEANING OF HETEROPHYLLY 155 



wrote in the seventeenth century, "there are some, which have 

 Leaves (besides the two first Dissimilar ones^) of Two Kinds 

 or Two distinct Figures \ as the Bitter-Sweety the common 

 Little Bell^ Valerian, Lady-Smocks, and others. For the Under 

 Leaves of Bitter-sweet, are Entire; the Upper, with two Lobes; 

 the Under Leaves of the Little Bell, like those of Fancy; the 

 Upper, like those of Carnation, or of Sweet-William^ 



We find parallels to the heterophylly of hydrophytes not only 

 among terrestrial Flowering Plants, but also in the case of the 

 distinct 'youth forms' of Conifers, and even more remotely 

 in the Chantransia stage of such Algae as Batrachospermum. 



The conclusion to be drawn from our very brief survey, which 

 only touches the fringe of the subject, is that heterophylly is so 

 widespread that no interpretation can be valid unless the con- 

 dition be treated broadly as a very general attribute of plant life, 

 rather than as a rare and exceptional phenomenon, for which 

 special and individual explanations will suffice. 



(3) THE INTERPRETATION OF HETEROPHYLLY 



To the earlier writers, such as Lamarck^, the problem of 

 heterophylly presented no difficulties. They regarded the sub- 

 merged or aerial type of leaf as representing a direct response, 

 on the part of the plant, to the medium. The work of the last 

 thirty years, has, however, rendered this simple conception 

 untenable; the theory that now holds the field accords a much 

 less prominent place to adaptation. The first observation that 

 cast doubt upon the idea that leaf form necessarily depended 

 directly on the milieu, was that of Costantin ^, who showed that, 

 in the case of Sagittaria, the aquatic and aerial leaves were 

 already distinguishable from one another in the submerged bud ; 

 he noticed auricles on a leaf which was only 2 to 3 mm. long. 

 In Ranunculus aquatilis, also, the leaves destined to be aerial 

 are differentiated in the bud. 



1 I.e. cotyledons. 2 Lamarck, J. B. P. A. (1809). 



3 Costantin, J. (18852) and (1886}. 



