232^ THE PLANT WORLD. 



In all such cases the power of endurance of the plant to an 

 excessive or defective action of any one force is very much modi- 

 fied by the presence of or action of others. Thus, in the matter 

 of the seeds, the endurance to extremes of temperatures is seen 

 to depend directly upon the amount of moisture present. 



Again the time element or the rapidity with which the in- 

 tensity of external conditions is changed is a basal factor in all 

 accommodation processes. As a plant accommodates itself to 

 live at unwonted temperatures, or in unaccustomed soils, but 

 little doubt exists that it undergoes changes in intricate struct- 

 ure, which, however, are not always to be demonstrated. So 

 long as the species remains under the new and strange conditions 

 the acquired structure Avill be retained from generation to gen- 

 eration, whether transmitted by cuttings or seeds. If the species 

 is returned to its original habitat or to the normal conditions in 

 which it originally grew, the acquired structures may persist for 

 a time, but in accordance with the power of accommodation 

 which originally brought them into existence, they \vill disap- 

 pear when the inducing factors are removed. 



The classical cultures of Bonnier made in the Alps and 

 Pyrenees, twenty years ago, furnish us with the bulk of the sys- 

 tematic information available as to the influence of elevation on 

 plants. From these it was seen that plants taken from lower to 

 higher altitudes, up to about 7,000 feet, and not exceeding the 

 optimum of the species, developed shorter internodes, the subter- 

 ranean parts of the plant were relatively much larger, the leaves 

 were smaller, mone deeply colored, and the flowers were also more 

 vividly tinted. 



N^o better illustration of the changes in structure shown by 

 plants, when accommodating themselves to habitats presenting 

 strange external conditions, can be found than those found in the 

 American water-cress (Roripa Americana), with which some ex- 

 tensive experimentation has been carried on. Originally taken 

 from the muddy bottom of Lake Champlain, where it gi-ows in 

 water at a depth of 1-3 feet, it had been gradually accommodated 



