MAXIMUM EXPOSURE OF SURFACE 279 



unit, which may correspond either to an entire cell or else merely to a 

 cell-branch, but which has in either event the same physiological value. 

 The radial walls of ordinary palisade-cells may then be regarded as 

 flanges which have extended right across the cell ; conversely, flanges 

 may be looked upon as incomplete partition-walls. This notion of a 

 palisade-unit may be further illustrated by comparing an arm-palisade- 

 cell to a group of normal palisade-cells which have become partially 

 fused. 



The case of Pinus may now be reconsidered in the light of the 

 conclusions arrived at in the preceding argument. It is evident that the 

 radial flanges in the peripheral arm-palisade-cells are of the same 

 nature as the irregularly disposed folds in the more deeply situated 

 cells, and that the same structural principle is responsible for the 

 existence of both ; in the one instance, however, it is only the presence, 

 of the flanges that demands an explanation, whereas in the other it is 

 necessary to account for their definite orientation as well. An enquiry 

 into the significance of palisade-tissue thus involves two questions, 

 namely first, as to what physiological benefits result from the 

 development of folds or partitions of any kind in photosynthetic cells, 

 and, secondly, as to what special advantage is gained by arranging of 

 these flanges or walls at right angles to the surface of the organ. The 

 present section is only concerned with the former question ; in these 

 circumstances the physiological significance of the irregularly disposed 

 flanges in the Pinus needle is the problem that lies nearest at hand. 



Let us first consider how the photosynthetic efficiency of a cell is 

 increased by the development of flanges. It is found not only that 

 chloroplasts invariably take up a peripheral position, but also that they 

 tend to spread themselves over any projections or fold of the wall that 

 may be present. Hence the physiological advantage of the flanged type 

 of photosynthetic cell consists principally in the fact that a larger 

 internal surface is provided for the display of chloroplasts. The principle 

 of maximum exposure of surface is thus not only responsible for the 

 breaking up of the primitive centralised chlorophyll apparatus into 

 separate corpuscles, but also governs the construction of the photo- 

 synthetic tissue as a whole. 



The increase in the number of chloroplasts that is rendered possible 

 by the development of flanges, or by the presence of their analogues, 

 the radial walls of ordinary palisade-tissue, is very considerable. If 

 the internal surface of a cell devoid of flanges be reckoned as equal to 

 100 superficial units, the corresponding value for Hanged cells may be 

 anything from 120 to 150. Typical values are: 145 for a three- 

 flanged pectinate cell of Bambusa Simonii, 115-135 for tabular cells of 

 Pinus stjlcestris with irregularly disposed folds, 127 for an H-shaped 



