252 The Living Plant 



which the stimuH act from a single direction, and therefore evoke 

 only one-sided responses. But some of the very same stimuli 

 may act in a diffused or all-around manner, becoming impressed 

 on the sensitive protoplasm of the plant through a change in 

 intensity; and in such cases the responses are all-sided or sym- 

 metrical. Thus the sleep movements of leaves, already con- 

 sidered, are of this nature, being a response to the change in in- 

 tensity of the circumambient light; and the same thing occurs 

 with some flowers, which close at night or in very dark weather. 

 Other flowers, e. g Tulips, are affected in like manner by changes 

 of temperature, opening as the weather grows warmer, and clos- 

 ing as it becomes cooler; and some evergreen leaves, notably of 

 Rhododendrons, rise and fall in this way even in winter. Such 

 responses are distinguished from the ordinary sort in scientific 

 terminology by the termination, nasty (photonasty, thermonasty, 

 etc.) ; and we may note by the way, that the responses due to a 

 free-swimming movement, as in the case of the antherozoids of 

 Ferns already described, are distinguished by the termination 

 taxis (chemotaxis, phototaxis, etc.). 



There are, furthermore, several other types of responses to 

 stimuli, some of them vastly important in connection with the 

 growth and development of plants. Thus, it has been claimed 

 that the strains set up by the swaying of stems back and forth, 

 whether in nature by winds, or in the laboratory under experi- 

 ment, serve as stimuli to the larger development of strengthen- 

 ing tissues in the places where the strains are most felt, thus pro- 

 ducing a needed enlargement at those places. It is perfectly 

 clear that the great knees which rise from the roots of the Bald 

 Cypress of the Southern Swamps and which probably are aerating 

 structures, are formed in response to the presence of water, for 

 they do not form at all when these trees grow in soil that is well- 

 drained. Other cases are known where the thickness of cell-walls, 

 the arrangement of tissues, the sizes of parts, and other structural 

 features are regulated by responses to well-known stinmli from 



