PHOTOSYNTHESIS IN LOWER PLANTS 151 



therefore in a separate air chamber. The photosynthetic prod- 

 uct is delivered to colorless parenchyma cells in the lower half of 

 the thallus. In Marchantia the thallus lies flat upon the ground, 



FIG. 83. Cross section through the thallus of Marchantia. j, stoma leading into a 

 relatively large air-chamber in which arise numerous cells with chloroplasts, k; 1, I, cells 

 forming partitions between the air-chambers; in, cells destitute of chloroplasts and with 

 numerous oblong pits; h, upper, and i, lower epidermis. (After Sachs.) 



and the photosynthesizing cells and stomata can function to best 

 advantage where we find them in the upper part of the leaf. 



In Sphagnum the leaf is one cell thick (Fig. 84), and the 

 photosynthetic cells are arranged 

 in the form of an open mesh- 

 work as seen from the surface, 

 and between them are large, 

 clear, water-storage cells having 

 minute pores through the outer 

 wall at the under surface through 

 which water can be imbibed. 

 As seen in the figure the photo- 

 synthetic cells are elongated 

 parallel with the length of the 

 leaf and are thus adapted to con- 

 duct away their own products. 



In Polytrichum there are 

 vertical chains of photosyn- 

 thetic cells, but there is no closed 

 upper epidermis, as shown in Fig. 85, and these cells are freely 

 exposed to the outer air excepting when the leaf, in danger of 



FIG. 84. Portion of leaf of Sphagnum, 

 in cross section on the left, and surface 

 view on the right, h, hole through the 

 wall; i, chlorophyll -bearing cells; /, 

 water-storage and water-conducting cells, 

 with annular thickenings at k. (After 

 Strasburger.) 



