270 PHOTOSYNTHETIC SYSTEM 



plants the cortical cells of which contain comparatively little chloro- 

 phyll on the other. The amount of oxygen evolved by the stems of 

 the leafy plants was always small, and often negligible, whereas all the 

 switch -plants exhaled considerable quantities of this gas. Thus the 

 amount evolved after three hours insolation was 1'6 c.c. per sq. cm. in the 

 case of Casuarina excelsa and 2'3 c.c. in that of Spartium monospermicm. 

 In comparing the photosynthetic energy of foliage-leaves which con- 

 form to the same general plan of construction, the relative chlorophyll- 

 content is a fairly safe criterion. Weber 145 has estimated the average 

 amount of dry substance produced photosynthetically by unit area of 

 leaf, in the course of a day, for various plants. Each species was found 

 to possess a " specific photosynthetic capacity," which was not, however, 

 further investigated by Weber. In these circumstances it seemed 

 desirable to the author to determine the numbers of chloroplasts per 

 unit area in Weber's plants, in order to ascertain if any definite relation 

 exists between photosynthetic activity and chlorophyll content. If both 

 quantities are reckoned as equal to 100 in the case of Tropaeolum 

 majus, the following values are obtained : 



-c T>i t Number of 



per unit area. ^ -, 



p per unit area. 



Tropaeolum majus, - - 100 1()0 



Phaseohts rnultiflorus, 72 64 



Ricinus communis, - - - - - 118 - 5 120 



Helianthus annims, 124"5 122 



The close agreement between the two sets of values is obvious and 

 striking. The slight discrepancies are no doubt due partly to the 

 imperfect experimental methods employed, and partly to differences in 

 the structure of the leaves and in the size of the chloroplasts. 



Most probably there is a specific photosynthetic capacity charac- 

 teristic of the individual chloroplast, which varies in different plants. 

 In other words, even where the chloroplasts of two plants agree in size 

 and shape, those of one species may display greater photosynthetic 

 activity under the same external conditions. Such specific differences 

 are, however, certainly of secondary importance in comparison with the 

 varying quantitative development of the chlorophyll apparatus, at any 

 rate as far as Phanerogams are concerned. 



o 



III. ANATOMY OF THE PHOTOSYNTHETIC SYSTEM. 



A. THE PRINCIPLE OF MAXIMUM EXPOSURE OF SURFACE. 



Palisade -parenchyma has been recognised as the most important of 

 specialised photosynthetic tissues, in the preceding section ; this tissue 



