262 PHOTOSYNTHETIC SYSTEM. 



assimilation of nitrogen may be mainly carried on by a special form 

 of tissue. Treub 136 has indeed shown, in the case of Pan/jivm cdule, 

 not only that the first demonstrable product of nitrogen-assimilation is 

 hydrocyanic acid a discovery which is of great importance for the 

 general theory of nutritive metabolism but also that this substance is 

 in all probability manufactured only by particular foliar epidermal 

 cells, and by certain idioblasts in the periderm and pith of the stem, 

 which are at the same time concerned with protein-synthesis. Further 

 investigation is required in order to determine how far the conditions 

 in Pangium at present an isolated case exemplify a widespread 

 phenomenon. In the meantime we have no choice but to focus our 

 attention upon the synthesis of carbohydrates that is, assimilation in the 

 strict sense [or photosynthesis, 1361 as it will henceforth be termed here] 

 and to make a detailed study of the tissues that are concerned with 

 this process. Other things being equal, the photosynthetic capacity of 

 a cell can be expressed with considerable accuracy in terms of its 

 chlorophyll-content ; the latter feature, in fact, provides a definite 

 criterion, which, whatever its imperfections may be, at any rate greatly 

 facilitates the task of delimiting the photosynthetic system. 



Just as a cell cannot be straightway identified as a mechanical 

 element, simply because it is thick-walled, so the photosynthetic system 

 does not necessarily include all the chlorophyll-containing cells of a 

 plant. A green cell should not in fact be regarded as a photosynthetic 

 element, unless photosynthesis is its principal function. This rule 

 must be all the more strictly enforced because photosynthesis is, in 

 accordance with its great physiological importance, one of the most 

 widespread of subsidiary functions. Neither the external nor the 

 internal conditions which determine the presence of chlorophyll are 

 of a very exacting nature. Chloroplasts may consequently occur in a 

 great variety of cells, such as ordinary epidermal elements or hairs, 

 bast-fibres or collenchymatous cells, elements of the parenchymatous 

 bundle-sheath and the like. While such " incidentally " photosynthetic 

 cells undoubtedly represent additions to the chlorophyll-apparatus of 

 the plant which are not altogether negligible ecologically, they are 

 nevertheless all of secondary importance from the physiological point of 

 view. Hence, in determining whether a given cell is a photosynthetic 

 element or not, it is not safe to rely solely upon its relative chlorophyll- 

 content. In order to rank as such an element, a cell must also fulfil 

 certain histological and topographical conditions and generally conform 

 to the principles of construction that are characteristic of the photo- 

 synthetic system as a whole. 



