59 



ECOLOGY 



septentrionalis) which always grow on land, and whose leaves are 

 .divided but not finely dissected, while other species (as R. circinatus) 



848 



849 



850 



852 



851 



853 



FIGS. 848-853. Leaf variation in an arrowhead (Sagittaria helerophylla): 848,3 

 representative air leaf; 849, 850, air leaves from plants in deeper water than those 

 bearing such a leaf as figured in 848; note the reduction or absence of the basal lobes; 

 851, 852, leaves from plants in deep water; 853, a submersed bladeless leaf (phyllode); 

 all the variants here figured may be found in a single vegetative colony connected (at 

 least originally) by underground stems; they may be found also on a single individual 

 at different developmental stages, the phyllode appearing first and the broad leaf last. 



always jyowj'n 



h avp dissected leaves; still other species 



(as R. multifidus or R. aquaiilis, figs. 854-857) have leaves of both sorts 

 and all kinds of intergradations, depending upon the habitat. The 



854 



857 



FIGS. 854-857. Leaf variation in the white water-buttercup (Ranunculus aquaiilis): 

 354, a water leaf, entirely submersed during development; 855, 856, leaves transitional 

 between air leaves and water leaves; 857, an air leaf. 



hypothesis is that the former species, characterized by slight plasticity, 

 e<x,ch have come from an ancestry comparable as to plasticity with the 

 present R. aquatiUs, Another hypothesis is possible, namely, that some 



