264 PHOTOSYNTHETIC SYSTEM. 



Simple pits are sometimes present; for example, in the rounded photo- 

 synthetic cells of succulents, in the mesophyll of the pinnae of Cycads, 

 and in the green parenchyma of the cladodes of JRuseus. Local thicken- 

 ings of the wall are rarely found in photosynthetic elements. Probably 

 the best-known instance is furnished by the palisade-cells of Cycas, 

 which are regularly provided with longitudinal thickening fibres ; these 

 evidently in the first instance enable the individual thin-walled palisade- 

 cells to withstand longitudinal compression, while they also protect the 

 whole tissue against radial pressure. 



B. CHLOROPLASTS. 



1. Form and structure of chloroplasts. 131 



Among the Algae, and particularly in the Chlorophyceae, the 

 chloroplasts assume a great variety of forms. The simplest condition 

 is that in which each cell contains a single large chloroplast ; the 

 latter is often saucer- or bowl-shaped, or tabular in form, and may 

 either be closely appressed to the wall (Palmellaceae, TJlva, Entcro- 

 morpha, Coleochaete) or else lie suspended in the middle of the cell- 

 cavity {Mougcotia). At the next stage of specialisation the chloroplast 

 is likewise solitary and flattened, but has developed marginal lobes or 

 frills, or has become perforated after the fashion of a gridiron or 

 lattice (Ocdogonium, Cladophora arcta). Occasionally it takes the form 

 of a narrow ribbon, which, again, may be straight, curved, or sinuous ; 

 the ribbon-shaped chloroplasts of Sjrirogyra are spirally twisted, and 

 in addition provided with inwardly projecting ridges. Zygnema 

 cruciatum, finally, has star-shaped chloroplasts : every cell has one 

 such stellate body at each end, the space between the two members 

 of a pair being bridged by a mass of cytoplasm enclosing the nucleus. 



Sharply contrasted with the great diversity of the forms assumed 

 by the chloroplasts in certain Chlorophyceae, is the uniformity of type 

 which characterises these structures in most of the groups of Algae, 

 and in almost all the Bryophyta, Pteridophyta and Phanerogams, where 

 they are commonly developed as small rounded, ellipsoidal, or (owing 

 to mutual pressure) polyhedral bodies, usually termed chlorophyll cor- 

 puscles (Fig. 107). In specialised photosynthetic cells these chlorophyll 

 corpuscles are always present in large numbers, forming a green peri- 

 pheral layer which covers the cell-walls more or less completely. 



Among the Higher Plants (that is, the groups from the Bryophyta 

 upwards) there are but few aberrations from the typical chorophyll- 

 apparatus composed of numerous "corpuscles." The genus Anthoceros 

 among Liverworts provides one of these exceptions. In this case every 

 photosynthetic cell of the gametophyte contains a single bowl-shaped 



