PHOTUSYNTIIKTIC < 'KLLS. 263 



//. PHOTOSYNTHETIC CELLS. 



A. SHAPE OF THE CELLS. CHARACTEE OF THE CELL-WALLS. 



The outward .shape of the elements that compose the photosynthetic 

 system varies within wide limits. In their simplest form photosynthetic 

 cells are isodiametric, with a tendency to become rounded off at the 

 edges. Much more frequently, however, they are elongated in a 

 definite direction and approximately cylindrical or tubular in shape; 

 the relation of this prevalent shape to the functions of the cells will 

 be explained later on. The orientation of green cells with reference 

 to the surface of the photosynthetic organ is also variable Sometimes 

 they lie parallel to the surface, in which case they may be extended 

 either along the principal axis of the whole organ, as in the leaves of 

 Elodea canadensis, Galanthus nivalis and Leucojum veriium, or athwart 

 this axis, as in Iris germanica, Erythroniv/m Dens finis and spp. of 

 Triton in and Gladiolus. Usually, however, they are placed at right 

 angles to the surface (rarely they are obliquely inclined), and in this 

 case receive the long-established name of palisade-cells. The ratio of 

 length (or height) to width is very variable ; some palisade-cells are 

 not much longer than their width, while in those of a more slender 

 type the height may exceed the breadth ten- or twelve-fold. Closely 

 related to the palisade-cell is the obconical type of photosynthetic 

 element or funnel-cell; this has an upper wide end, which nearly 

 always abuts immediately against the epidermis, and a narrow lower 

 end, which generally rests upon elements of the spongy parenchyma. 



A remarkable modification, and one which is of great importance 

 for the understanding of the palisade form, is the so-called arm-palisade- 

 cell ; in this case the palisade, instead of consisting of entire cells, is 

 made up of groups of cell-branches or -arms. It should be remarked 

 that the palisade-cell is merely a particular variety characterised by 

 its special orientation of the generalised tubular photosynthetic element; 

 this fact must not be overlooked when one seeks to obtain a physio- 

 logical explanation of the shape and arrangement of palisade-cells. 

 From the arm-palisade cell the transition is easy to the tabular photo- 

 synthetic elements that are found in various Conifers (Pinus, Cedrus) 

 and Gramixeae ; these are polyhedral cells, with more or less pro- 

 nounced flanges projecting inwards from the cell-wall. Finally, we 

 may include in this list of cell-shapes the spongy parenchyma type, 

 which has numerous radiating branches, and may even approach a 

 stellate form, though as a matter of fact photosynthesis is normally a 

 subsidiary function for cells of this character. 



The walls of photosynthetic cells are in general thin and soft. 



