Minnesota Plant Life. 



449 



is submerged and part exposed, the leaves of the siil)merged 

 portion may be finely dissected, while the leaves of the emerg- 

 ent area are simple and but slightly notched. Of this the 

 water bnr-marigold is an example. Again, the leaves are all 

 submerged and ribbon-shaped, as in the water eel-grass, or 

 they may be quill-shaped, with air chambers, as in the quill- 

 worts and water lobelias. When some of the leaves are float- 

 ing and others are submerged, the floating leaves are often 

 larger and broader than the submerged. Thus, in the float- 

 ing arrowhead the natant leaves are arrowhead-shaped, while 

 the submerged leaves are grass-like ; or, as in the water-shield 

 (not, however, the Minnesota variety), the floating leaves are 

 shield-shaped and the submerged leaves are finely dissected. 



In other instances, 

 as in the water-lilies, 

 the dilTerence be- 

 tween the submerged 

 and floating leaves is 

 not so great, but a 

 slight variation in 

 texture is not hard 

 to observe. The fine 

 dissection of the sub- 

 merged leaves of so 

 many aquatic flow- 

 ering plants is evi- 

 dently an adaptation, by which a considerable absorptive power 

 is added to the ordinary starch-making function of the leaf. Such 

 finely dissected leaves are easily maintained under water, but 

 they would be likely to shrivel on account of too rapid evap- 

 oration if developed in many land forms. The floating leaves, 

 like those of the planktonic vegetation, have adaptations against 

 moistening, so the upper side of the shield-shaped leaf of the 

 Minnesota water-shield will be found to be waxy and dif^cult 

 to wet. The generally oval or circular shape of floating leaves, 

 or floating stems like those of the duckweeds and swimming 

 Riccias, may be regarded as a response to the equal lapping of 

 the waves against the edges of the leaf from all directions. 

 These floating leaves, or stems, are very often purple on the 

 under side, so that the surplus light is converted into heat. 

 30 



Fig. 222. — Birch trees along a lake shore. Bar vegeta 

 tion in background. After photography by Williams. 



