4 THE LIVING PLANT 



through the cortex to the water-conducting elements of the 

 vascular cylinder, and this supplies the shoot system. The 

 shoot system, no less highly organized, is, in the first instance, 

 concerned with the manufacture of food, carbohydrate, fat and 

 protein. In this connection the leaf, a marvel of organization 

 with its chlorophyll apparatus supported by the network 

 of veins which also are the conduits for the conveyance of fluid 

 raw materials and for the elaborated products, and with its 

 mechanism for the regulation of gaseous interchange is the 

 great synthetic factory, building up food apparently with the 

 greatest ease and certainly with remarkable rapidity. 



In due season reproduction takes place. Of the problems 

 here involved the secretion of nectar, when it obtains ; the facts 

 of fertilization and the stimuli which invoke the segmentation 

 of the egg ; the transmission of hereditary characters ; the 

 reconstruction of the food destined for the use of the off- 

 spring ; and the mechanisms of dispersal, are of fundamental 

 importance. 



Of the various aspects of the life of the plant outlined in 

 the foregoing rather breathless account, it is appropriate on 

 the present occasion to consider those associated with meta- 

 bolism, the making of food and the procurement of energy. 

 The various laws and conceptions involved have either been 

 explained in the first volume of this work or are considered 

 as circumstances demand in the following chapters. To this 

 there is one exception : the determination of the concentration 

 of the hydrogen ion is a very delicate measure of the reaction, 

 acid or alkali, of a fluid and is invaluable in investigations 

 where exact comparisons are required. An explanation of 

 the principles involved is given here rather than on the occasion 

 of its first mention in the following pages where it would 

 unduly interrupt the narrative. 



THE HYDROGEN ION CONCENTRATION. 



A normal solution of any acid or salt is defined as one 

 containing one gram of hydrogen or its equivalent dissolved 

 in one litre of water. According to this definition, the weights 

 of hydrochloric, nitric, acetic and any other monobasic acid 

 contained in a litre of normal acid would be the respective 

 molecular weights in grams, namely HC1 = 36-5, HNO 3 = 63, 



