PH0T0SYNTHET1C SYSTEM IN BRYOPHYTA & ALGAE 297 



istic shade plants possess typical palisade-tissue, the palisade-cell must 

 not be regarded as a form of photosynthetic element that has been 

 evolved in adaptation to intense illumination, although the development 

 of the tissue is undoubtedly favoured by bright light, as Stahl himself 

 first discovered. Similarly, it cannot be argued that the shape and 

 orientation of palisade-cells depends primarily upon the direction of the 

 incident light. 152 



V. THE PHOTOSYNTHETIC SYSTEM AMONG BRYO- 

 PHYTA AND ALGAE. 



The photosynthetic system reveals the dominating influence of the 

 two principles of maximum exposure of surface and expeditious trans- 

 location in various degrees among the Bryophyta and Algae, just as it 

 does in the case of higher Plants; on account of certain peculiarities, how- 

 ever, the lower green plants are most conveniently treated separately. 



Flo. 123. 

 T.S. through the thallus of Marchantia. sp, air-pore ; chl, photosynthetic filaments. 



Among thallose Liverworts it is more especially the Marchantiales 

 that are provided with a highly differentiated photosynthetic system 

 (Fig. 123). Here the upper surface of the thallus is divided into a 

 number of lozenge-shaped areas, each of which is underlain by a sub- 

 epidermal air-chamber; these chambers are separated from one another by 

 partitions, composed of a single layer of cells, and corresponding to the 

 boundaries of the lozenge-shaped areas. The epidermis is perforated over 

 the centre of each chamber by a wide circular air-pore. From the floor 

 of the chamber arise numerous erect branching filaments, composed of 

 cells containing abundant chloroplasts. These structures, which remind 

 one of Algal filaments, constitute the actual photosynthetic tissue of 

 the plant. The basal element of each filament is funnel-shaped, and 

 represents a collecting cell, which serves to transmit the synthetic 

 material manufactured in the upper cells of the filament to the under- 

 lying colourless parenchyma. The entire layer of filaments thus agrees 

 with a palisade-tissue as regards the course of the current of migrating 



