﻿XEROFOTIC MOVEMENTS IN LEAVES 



(with eight figures) 



Definition. — The word xerojotic, expressing the ideas of dry- 

 ness and light, gives a clue to the meaning the word is intended 

 to connotate, namely, dryness caused by light. Xerofotic move- 

 ments, therefore, are paratonic movements resulting from certain 

 drying effects produced through the action of light. They are 

 manifested by an upward bend in the leaflets or a curling or rolling 

 upward of the blade. 



Mechanism. — The side of a structure facing the sun becomes 

 warmer than the opposite side. Under these conditions, other 

 things being equal, there is a relatively greater loss of water from 

 the exposed side. When the water is not replenished as fast as it 

 evaporates, a greater lowering of the turgidity in the cells of the 

 upper side ensues. The greater pressure exercised by the more 

 turgid cells of the lower side causes the structure to bend or roll 

 in the direction of the exposure. In the case of the most con- 

 spicuous examples, the leaflets of leguminous plants, the result of 

 this action of light is an upward movement of the leaflets. The 

 base of the leaflet is the seat of the differential turgidity. 



No amount of heat or rapid transpiration is sufficient to cause 

 the xerofotic movement unless there is also a difference in turgidity 

 caused by one-sided illumination. This movement is entirely 

 distinct from the collapse or wilting caused by too great a drying, 

 and from the photeolic, or so-called sleep, movements. 



Classification. — Two kinds of xerofotic response were 

 observed: the localized response, in which the differential turgidity 

 is largely confined to a small region, as, for example, the pulvini 

 of leguminous leaflets; and the generalized response, in which the 

 differential turgidity is spread over the leaf, causing the blade to 

 curl or roll upward. Cases in which the leaves roll downward 

 or underneath must be considered as wilting and not as xerofotic 

 responses. 



