88 VEGETABLE PHYSIOLOGY 



nation, affecting successive generations of plants through 

 an enormous length of time, may have impressed upon the 

 protoplasm a peculiar rhythm of greater and less general 

 activity, which has become ultimately automatic and inde- 

 pendent of the immediate surroundings. Of this the vary- 

 ing action of the roots may be a particular expression. 



It is remarkable, however, that very young plants do 

 not exhibit this diurnal variation, but they gradually 

 acquire the power of doing so as they develop, subject as 

 they are under normal conditions to the alternation of light 

 and darkness. In many cases, again, the diurnal periodi- 

 city is not manifested at all. 



The effect of the periodic alternation of light and dark- 

 ness cannot in any case have been originally appreciated by 

 the roots, as they are implanted in the soil and so escape 

 its influence. If it was originally due to such variations, 

 these must have been impressed upon the general orga- 

 nisation of the plant. 



TRANSPIRATION. The modified evaporation by which 

 the protoplasts get rid of water and enable the contents of 

 their vacuoles to be continually renewed takes place ulti- 

 mately from the surfaces of all the succulent parts of 

 plants, and to a less extent from portions of the exterior 

 which are covered by a layer of cork. Like the activity 

 of the absorbing organs of the root, it is essentially a vital 

 process and is regulated by the protoplasm of the cells 

 which take part in it. As we have seen, it is usually spoken 

 of as transpiration. 



It is easy to demonstrate the fact of its continuous 

 existence during daylight by enclosing a plant, or part of 

 one, in a dry glass vessel which can be closed so as to 

 admit no air. Very soon the surface of the glass becomes 

 covered by a fine dew, which is the condensed vapour that 

 has escaped from the plant. The same thing may be seen 

 when a vigorous plant is covered over by a bell-jar, the 

 water condensing copiously upon the sides of the latter. 



A more elaborate method of demonstrating transpiration 



