NUTRITIVE METABOLISM 295 



enzymes, by means of which extracellular digestion is possible, and certain 

 animals, on the other hand, such as the Tape-worms and many Infusoria, 

 absorb nutriment in solution through the outer body wall. In Protozoa 

 undigested fragments are thrown out again through a permanent aperture 

 (mouth or anus) or through a temporary opening made in the body wall, 

 and the same occurs when the plasmodium of a Myxomycete excretes 

 undigested solid particles. No such excretion is necessary in typical plants, 

 owing to the fact that the whole of the food is absorbed in solution, while 

 the final excretory products of metabolism are for the most part volatile 

 or soluble, and hence are able to escape from the enclosed plant cells to 

 the surrounding air or water. 



Higher plants exhibit a large amount of differentiation of labour as 

 regards the parts which the various organs play in obtaining nutriment. 

 In terrestrial plants the roots which attach them to the soil absorb 

 the nutritive substances present in the latter, while the leaves are mainly 

 concerned in the absorption of gases from the air. In order to obtain food, 

 a certain amount of energy must be expended even in plants which do not 

 hunt down their prey as animals do, for the development of absorptive 

 organs and the penetration of the root into the soil involve a certain 

 expenditure of energy. The same occurs when enzymes are secreted ; 

 indeed the plant frequently sacrifices a large proportion of its food in order 

 to obtain the energy necessary to construct the essential plastic and 

 constructive materials. 



Carbon compounds provide both energy and materials for growth, so 

 that it is imperative that an adequate supply of the appropriate organic 

 food shall always be at the plant's disposal. In addition to carbon, 

 hydrogen, oxygen, and nitrogen, plants contain small traces of certain 

 other essential elements, which form part of the ash left behind on burning. 

 These are potassium, magnesium, phosphorus, sulphur, iron, and, except in 

 Fungi and certain Algae, calcium also. These ash constituents are always 

 present not only in a piece of wood, but in every cell and every particle of 

 protoplasm. Silicon, sodium, and other elements may also occur, and 

 although these are not necessary to the plant, they may occasionally be 

 present in great abundance. Much more than the necessary minimum of 

 the essential elements is commonly absorbed, and hence the ash may form 

 as much as 20 per cent, of the dry weight in a few cases, although the 

 presence of 2 to 4 per cent, by weight of ash suffices for all requirements. 

 Only very small amounts of sulphur and iron are necessary, but never- 

 theless, in the absence of these minute quantities the vital activity of the 

 plant comes to a standstill, just as a watch stops when the tiniest wheel is 

 removed which forms an essential part of its working mechanism. 



The analysis of the ash constituents gives no indication as to the form 

 in which the mineral elements are retained by the living organism, but it is 



