CHAPTER II. FUNCTIONS OF THE HIGHER PLANTS IN 



DETAIL. 



89. It has been already pointed S a f fcF , ood 



-, i t ^ i ^i i- < , Materials. 



out that the higher plants absorb their supplies of mineral 

 salts and water mainly by means of the root-hairs, each of 

 which is merely the prolongation of a single, living, epidermal 

 cell, provided with a thin wall of cellulose and containing 

 protoplasm, a nucleus, and cell-sap. In all essential details, in 

 fact, it resembles our algal cell except that the green chlorophyll 

 corpuscles are absent. In such a root-hair, therefore, the 

 assimilation of a carbohydrate from inorganic materials cannot 

 take place, but respiration and the absorption of mineral 

 substances in solution can take place, as in the alga. 



The chemical constituents of the plant body can be found 

 by analysis. Water in considerable quantities always occurs 

 In a living plant, and in turgid herbaceous plants water 

 may amount to as much as 90% of the entire weight of 

 the plant. About half the dry weight of a plant is made up of 

 carbon. 



In Part II we have seen that a large number of additional 

 substances enter into the composition of the protoplasm, but 

 other substances also occur and practically all known elements 

 have, at one time or another, been detected in plants. 



In order to discover which of all these substances are Essential 

 absolutely essential and in what quantities they are required Substancss. 

 for the growth and development of various plants, the latter 

 must be grown in pure water to which only known quantities 

 of known substances are added. By such water-culture ex- 

 periments it has been discovered that the following are indis- 

 pensable for the higher plants : 



Carbon. Phosphorus. 



Hydrogen-. Potassium. 



Oxygen. Magnesium . 



Nitrogen. Calcium. 



Sulphur. Iron. 



All of these substances, excepting potassium, magnesium 



and calcium, are believed to enter into the composition of 



the protoplasm itself. The calcium appears to be often 



useful in combining with poisonous substances, e.g. oxalic 



acid, which are then deposited in the plant in a harmless 



form. Of some of these essential substances only very small Quantities 



quantities are required, but if that small quantity is absent te i u!l ' ei1 - 



