242 THE SIMPLEST LAND PLANTS 



like the rhizoids of Pellia, increasing the absorptive 

 surface. 



Secondly, there is a highly differentiated double 

 conducting tissue system — the vascular system — ^run- 

 ning throughout the plant, and consisting of a water- 

 conducting and an organic food-conducting portion. 

 We have already seen that something of the kind, 

 though not very highly differentiated, exists in the 

 stalk of the sporogonium in Mosses. This double 

 conducting system is a necessity in bulky plants in 

 which the regions of water absorption, evaporation, 

 photosynthesis and growth are all localised. 



Thirdly, the photosynthetic organs, the foliage leaves, 

 are typically plates of tissue several cells thick, in 

 which the photosynthetic tissue (mesophyll) is inter- 

 penetrated by intercellular air spaces and is covered 

 externally — ^in common with the whole shoot — ^by a layer 

 of cells {epidermis) without chloroplasts, whose outer 

 walls, in contact with the air, develop a waterproof 

 layer, the cuticle. The epidermis and cuticle are pierced 

 by pores {stomata), each surrounded by a pair of special 

 cells {guard cells) containing chloroplasts which regulate 

 the size of the pore (Figs. 50, 51). 

 ( Most of these characters are foreshadowed in the 

 Mosses. In the sporogonial wall, for instance, we 

 met with the essential characters of the mesophyll, 

 epidermis and stomata of the foliage leaf ; and a 

 cuticle is developed more or less strongly both on the 

 outer surface of the sporogonium and elsewhere. But 

 the cuticle is neither so universal nor (usually) so 

 strongly developed in Mosses and Liverworts as in 

 the Vascular Plants. Again, we saw that rudiments 

 of the double conducting system are found in the 

 stalk of the sporogonium, which is in all respects the 



