4 THE LIVING PLANT 



facts of fertilization and the stimuli which invoke the seg- 

 mentation of the egg ; the transmission of hereditary charac- 

 ters ; the reconstruction of the food destined for the use of the 

 offspring ; and the mechanisms of dispersal, are of fundamental 

 importance. 



Of the various aspects of the life of a higher plant outlined 

 in the foregoing fleeting account, it is appropriate on the 

 present occasion to consider those associated with metabolism, 

 the making of food and the procurement of energy. 



But before beginning this, it is not altogether out of place 

 to draw attention to certain features which, although obvious, 

 are sometimes overlooked. The investigation of the meta- 

 bolic processes of plants is pursued by growing the plant in 

 controlled conditions and measuring the effect of a selected 

 factor by-varying that factor only. A positive result will be 

 the resultant of the combined action of the factors concerned, 

 and although it may be of the greatest value, it, obviously, 

 can give no information of what happened in the living 

 cells. To ascertain this, other methods, chiefly chemical 

 and physico-chemical, are used : attempts may be made 

 to bring about in the test tube processes which occur in 

 the plant, or analyses may be periodically made of the con- 

 tents of the tissues where metabolic activity is taking place 

 in order to find the sequence of the chemical change. The 

 former method is of value in giving information of the chem- 

 ical possibilities and has led to great advances in knowledge ; 

 but it does not follow that because this or that can be accom- 

 plished in vitro, the same events happen in vivo, for the con- 

 ditions obtaining in the active living cell would appear to be 

 of infinite complexity ; metabolic activities, both catabolic 

 and anabolic, oxidative and reductive, taking place concur- 

 rently, and thus various actions, reactions and digressions 

 may occur which never happen in the relatively simple con- 

 ditions obtaining in the test tube. 



The second method, that of analyses of cell contents, has 

 yielded valuable information about final products, but so far 

 has thrown but little light on the sequence in chemical change 

 since the intermediate phases may be transitory in the plant, 



