270 



PHOTOSYNTHETIC SYSTEM. 



radical leaves of species of Sempervivum. Here the photosynthetic 

 system is made up of radial plates of parenchyma, which extend 

 parallel to the long axis of the leaf. Each plate consists of a single 

 layer of cells, and is separated from the adjoining plates by air-spaces. 

 The chloroplasts are normally confined to the walls bounding these 

 intercellular spaces, while the transverse septa in each plate are left 

 entirely bare. In the case of typical palisade-tissue, also, it is not 

 unusual to find the chlorophyll corpuscles adhering chiefly to those 

 strips of the anticlinal walls which are in contact with air-spaces 



(Lcucojum vernum, Echinops exaltatus, 

 Centaur ea macrophylla, Cirsium pan- 

 nonicum, C. palustre, Fig. 107 b). 



Just as certain regions of the wall 

 in photosynthetic cells, are, as it were, 

 sought out by the chloroplasts, so 

 others seem to be avoided by these 

 structures. In palisade-cells, accord- 

 ing to the author's own observations, 

 the walls that are generally devoid of 

 chloroplasts are those which lie athwart 

 the paths normally followed by streams 

 of diffusing material. It is, of course, 

 the periclinal walls that are constantly 

 traversed, on the one hand, by currents 

 of water flowing into the green cells 

 from the epidermis or the water-tissue, 

 and, on the other hand, by outward 

 currents of synthetic products travel- 

 ling towards the spongy mesophyll or the conducting parenchyma. It 

 must not be supposed that the periclinal orientation of the walls is the 

 determining factor in such cases. Where, namely, the upper end of a 

 palisade-cell projects into an air-space, the freely exposed tangential 

 wall is occupied by chlorophyll-corpuscles (Fig. 107 c); in curved 

 palisade cells, on the other hand, no chloroplasts are found on the 

 inner tangential walls, even where the latter are obliquely inclined, 

 or almost perpendicular to the surface (Fig. 107 a). 



The influence of the nucleus upon the position of chloroplasts must 

 be included among the internal factors that regulate the arrangement 

 of these bodies. A relation of this kind is often clearly demonstrable 

 even among the Algae. Thus in Mougeotia the nucleus always lies 

 close up against the axile chloroplast, near the centre of one of its 

 broad faces. In some species of Spirogyra the nucleus likewise 

 adheres closely to the ribbon-shaped chroinatophore, while in others, 



Fig. 107. 



Arrangement of chlorophyll corpuscles in 

 palisade-cells. A. Curved palisade-cells of 

 Scilla bifolia. B. Palisade-cells of Cirsium pan- 

 nonicum (in T.S.). C. Palisade-cells of Brassica 

 Jiapa. For explanation see text. 



