1 62 HETEROPHYLLY [CH. xi 



with in some detail in Chapter xxvm, the ancestral leaf of this 

 family was band-shaped, while the oval or sagittate blade, 

 or 'pseudo-lamina,' is a later development a secondary ex- 

 pansion of the distal region of the sheath or petiole. The 

 submerged youth-leaves of this family would thus represent a 

 reversion to phylogenetically older forms. 



If the interpretation of heterophylly indicated in the present 

 chapter holds good at all widely, the teleological view of the 

 submerged leaf must be considerably modified. The present 

 writer would like to suggest that, for the old conception of 

 heterophylly as induced by aquatic life, we should substitute 

 the idea that such a difference between the juvenile and mature 

 forms of leaf as would render the juvenile leaf well suited to 

 life in water, has been in many cases one of the necessary pre- 

 liminaries to the migration from land to water, and that the 

 aquatic Angiosperms thus include, by a process of sifting 1 , 

 those plants whose terrestrial ancestors were endowed with a 

 strong tendency towards heterophylly 2 . 



1 Guppy, H. B. (1906) first emphasized the fertile idea that the 

 habitats of plants were determined by their peculiarities of structure, and 

 not vice versa. In relation to the occurrence of plants with buoyant seeds 

 and fruits in water-side stations, he writes, " there are gathered at the 

 margins of rivers and ponds, as well as at the sea-border, most of the British 

 plants that could be assisted in the distribution of their seeds by the agency 

 of water. This great sifting experiment has been the work of the ages, 

 and we here get a glimpse at Nature in the act of selecting a station." 



2 In addition to the references mentioned in this chapter, MacDougal, 

 D. T. (1914) and Shull, G. H. (1905) may also be consulted; the results 

 recorded in these papers emphasize the difficulty and complexity of the 

 problem. 



