25 THE USES AND ORIGIN 



Plants respond, however, to more definite external 

 changes than those dependent upon seasonal mutations. 

 Their flowers open and shut at particular hours of the 

 day, in accordance with the varying amounts of heat and 

 sunlight falling upon them. They grow more rapidly 

 by night than hy day, though as a general rule the activity 

 of their internal changes is closely related to the degree 

 of heat to which they are subjected. Again, whilst they 

 generally grow best in directions where they meet with 

 most air and light (not because of the latter agency, but 

 rather on account of the heat which goes with it), many 

 of them will, in the course of a few days or within 

 shorter periods, bend very perceptibly, so as to bring them- 

 selves more under the influence of this latter agent. 



Amongst some representatives of plant life, the corre- 

 spondence between internal and external changes is 

 undoubtedly less obvious than in many of the instances 

 just referred to. Thus is it with the black or grey film 

 of Lichen which marks as with a patch of paint the damp 

 surface of some weather-beaten rock. Yet, watch it care- 

 fully from time to time, and, even in this lowly form of 

 life, responsive though sluggish changes may be detected, 

 sufficient to remove it from the category of inanimate 

 things to which the rock itself belongs. 



The comparative complexity of life exhibited by mem- 

 bers of the vegetable kingdom is, however, small ; and 

 for this two principal causes may be cited. 



(1.) As a rule to which there are only few though 

 interesting exceptions, to be mentioned further on they 

 subsist on inorganic materials, deriving their food from 

 the gaseous or dissolved mineral elements existing in the 

 air or water with which their surfaces are bathed. In 

 their natural or healthy state plants decompose carbonic 

 acid, fixing its carbon and setting free its oxygen. They 



