CHLOROPHYLL-CONTENT 275 



cell of the spongy parenchyma. As the palisade-cells outnumber the 

 spongy elements approximately in the ratio of two to one, the palisade- 

 tissue is furnished with about 403,200 chloroplasts per sq. mm. of 

 surface, whereas not more than 92,000 per sq. mm. are present in the 

 spongy parenchyma. The former tissue thus contains 82 per cent, of 

 the total number of chloroplasts present. In the following table the 

 corresponding values are set forth for various other plants : 



Fraoaria elatior, ----- 



Pulmonaria officinalis, - - - - 



Ricinus communis, - 



Brassica Rapa, ----- 



Galeopsis Tetrahit, ... - 



Tropaeolum majus, . - . 



Helianthus animus, - 



Phaseolus multiflorus, - 



Bellis perennis, ----- 



The palisade-tissue therefore contains at least twice, very often three to 

 five times, and in extreme cases six times as many chlorophyll corpuscles 

 as the spongy parenchyma. These two tissues undoubtedly differ in a 

 similar degree in regard to their photosynthetic activities ; the contrast 

 in this respect is in fact probably even greater, for a twofold reason. 

 In the first place, the palisade-tissue, being situated next the upper 

 side of the leaf, is much better illuminated than the spongy parenchyma 

 which it overlies and shades ; secondly, the removal of synthetic pro- 

 ducts proceeds as is shown below in more detail much more rapidly 

 in the palisade-cells than it does in the spongy tissue. 



The generalisation which may be founded upon all these facts, is to 

 the effect that the palisade-layers constitute the special photosynthetic 

 tissue of the ordinary dorsiventral leaf. The same conclusion can be 

 drawn from a comparative anatomical study of the axial structures in 

 those leafless plants that have transferred their photosynthetic system to 

 the stem. Equisetum, Ephedra, Asparagus, Casuarina, Spartium, species 

 of Genista, etc., belong to this class. In such "switch-plants" the cortical 

 parenchyma of the younger branches assumes the character of a typical 

 palisade-tissue, a circumstance which shows that this type of tissue 

 does not merely represent a morphological constituent of the foliage-leaf, 

 but is really the essential anatomical component of highly organised 

 photosynthetic organs in general. 



Pick 144 has demonstrated the great photosynthetic activity of pali- 

 sade-cells by actual gasometric analysis. He carried out comparative 

 estimations of the evolution of oxygen, in light, from the green branches 

 of leafless switch-plants on the one hand, and from the twigs of leafy 



