59° 



ECOLOGY 



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

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



853 



I 



Figs. 848-853. — Leaf variation in an arrowhead {Sagittaria helerophylla) : 848, a 

 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, 85 2, 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 grow in the water and have dissected leaves; still other species 

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

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



Figs. 854-857. — Leaf variation in the white water-buttercup (Ranunculus aquatilis): 

 854, 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, 

 each have come from an ancestry comparable as to plasticity with the 

 present R. aquatilis. Another hjrpothesis is possible, namely, that some 



