294 



PHOTOSYNTHETIC SYSTEM 



in flattened structures, however, such as ordinary leaves, phyllodes and 

 cladodes, the shape of which ensures the most effective illumination of 

 the internal tissues, the differences of light-intensity at different points 

 in the interior are sufficient to exert an appreciable influence upon the 

 disposition of the specialised photosynthetic parenchyma, more especially 

 where the latter is developed as palisade-tissue. In the case of many 

 more or less erect leaves which receive approximately the same amount 

 of light on either side, the chances of effective illumination of the 

 interior are so good, that the entire thickness of the mesophyll, from 



Fig. 121. 

 T.S. through the isobilateral leaf of Scabiosa ucrainica. After Heinricher. 



one epidermis to the other, is made up of tubular photosynthetic 

 cells or palisade-tissue, with at most a single layer of intermediary cells 

 in the very middle. According to Heinricher, this type of structure is 

 exemplified by the leaves of Scabiosa ucrainica (Fig. 121), Moricandia 

 arvensis, Ckelone Torreyi. Turgenia latifolia, etc. In a slightly different 

 form of isobilateral leaf the spongy tissue is rather more extensive ; it 

 makes up the central portion of the mesophyll, while the peripheral 

 palisade-tissue is equally developed on both sides of the leaf. 



The connection between the disposition of the specialised photo- 

 synthetic tissue on the one hand, and the intensity of illumination on 

 the other, is still more obvious in the case of typical dorsiventral 

 leaves, which are extended at right angles to the direction of the most 

 intense diffuse light, and which consequently possess distinct " light " 



