DISTRIBUTION AND ADJUSTMENT 



101 



Some plants growing in the intense light of the marsh and 

 prairie have very interesting habit adaptations that avoid the 

 intense light and heat of midday. The upright position of 

 the leaf in the grasses, the iris, and cat-tails accomplishes this, 

 for the edge or tip of the leaf is presented to the midday glare, 

 the broadside of the leaf catch- 

 ing the less intense early 

 morning or late afternoon sun. 

 The common lettuce (Fig. 63) 

 and the compass plant, Sil- 

 phium laciniatum, both have 

 the habit of turning their 

 leaves so that they are in a 

 vertical position at noon, the 

 tips north and south, one edge 

 up, the other down, so show- 

 ing the edge rather than the 

 broad side to the noon light. 



No single line of demar- 

 cation in the distribution of 

 animals is as^ clear cut as that 

 between the water animals 

 the water breathers as they 

 are commonly called and the 

 air breathers. Of course in each case the oxygen in the air is the 

 important element taken, but in the water forms this is absorbed 

 from the supply dissolved in the water, while the others take it 

 directly in the respired air. Probably in the course of evolution 

 most animal as well as plant life was aquatic in its origin. The 

 hydrophyte and the hydrozooid are the primitive types, and 

 these early forms lived not only where water was abundant 

 but they lived under water. Such an existence necessitates on 

 the part of complex forms some special device for taking the 

 needed oxygen, for every living thing must have this essential 

 gas since it is only by constant oxidation that the energy supply 



FIG. 63. A compass plant, wild let- 

 tuce, Lacluca scariola. 



