152 



PLANT-LIFE 



lation and overflow. If there is to be a circulation 

 of sap, some water must be allowed to evaporate from 

 the leaves in order tbat water laden with fresh materials 

 may take its place. Full provision is made for this 

 evaporation. On the under-surface of a Male Fern leaf 

 the epidermis is pierced with little pores, or stomata, 

 as shown in Fig. 49, so constructed that they may open 

 or close to facilitate or prevent evaporation. Usually 

 they are open on a hot day, when much water passes 

 out of the leaves in the form of vapour. But the 

 stomata serve other purposes; through them carbon 

 dioxide is admitted and passed on to the busy alchemists, 

 the chloroplasts ; and through them, too, superfluous 

 oxygen, and also carbon dioxide resulting from internal 

 combustion, pass out into the air. In the Pteridophytes 

 the vascular system ramifies throughout the plant, 

 through roots, stems, and leaves, and the sporophyte is 

 in every respect furnished for independence. 



It may be well at this stage to contrast the life-cycles 

 of a Moss and a Fern : 



^1 



CO 





