Adaptation to Environment. 



327 



have been able to thrive away from the sharp competition 

 which prevails among plants in more favorable situations. 



It is not surprising to find that plants which have shown 

 such great adaptability to environment as have the xero- 

 phytes and the 

 hydrophytes, are 

 able to undergo 

 radical modifica- 

 tions in form and 

 structure even in 

 the lifetime of a 

 single individual, 

 if the conditions 

 under which they 

 are growing be- 

 come much al- 

 tered. Ranuncu- 

 lus fluitans, for 

 instance, which has its leaves divided into fine filamentous 

 segments when growing in the water, produces leaves with 

 much broader divisions when growing on the land (Fig. 174). 

 So, too, the submerged leaves of Cabomba Caroliniana are 

 finely divided, while the floating leaves are entire (Fig. 175). 



The stems of Callitnche stagnalis when growing in the 

 water have intercellular spaces occupying about half of 

 their volume, while the intercellular spaces of stems of the 

 same species found growing on the land are relatively 

 quite small (Fig. 176). 



Having become aware of the ability of plants to undergo 

 adaptive changes, we are prepared to understand in our 

 study of the plants of different regions, and of past ages, 

 how plants have been able to migrate from one latitude to 

 another without suffering extinction. 



Fig, 176. 



Cross section of stems of Callitriche stagnalis. A, 

 froni a plant growing in water; B, from a plant grow- 

 ing on land. Note how much smaller are the inter- 

 cellular spaces and how much thicker the epidermis 

 in the latter form. X 60. After SCHEN'CK. 



