so 



PLANT LIFE. 



Finally the wall formed between the two guard-cells splits, 

 and under certain conditions the two guard-cells become 



Fig. g.— Epidermis of underside of leaf of Virginian Spiderwort [Trades- 

 cantia virginica). A, seen from the surface; B, in cross section 

 through the leaf (x 240). (From Strasburger.) 



concave towards the median split, thus forming an opening 

 through the epidermis which opens into a large air-cavity, 

 situated in the fundamental tissue below. The opening 

 between the two guard-cells is the stoma. The details 

 connected with the formation of stomata are often much 

 more complicated than described above. Stomata are often 

 very numerous on the green parts of plants, as leaves, and 

 are for the purpose of effecting an interchange between the 

 gases contained in the plant and the external air. Carbonic 

 dioxide, as a food cortstituent, is taken in through the 

 stomata, as is also oxygen for respiratory purposes, and the 

 carbonic dioxide is exhaled ; a portion of the water contain- 

 ing required substances in solution taken in by the plant 

 also escapes into the air through the stomata ; this last act 

 is termed transpiration. Water required by the plant does 

 not enter through the stomata. In addition to stomata. 



