FORM AND POSITIOX OF THE TEANSPIRING LEAVES AND lUiAXCHES. 



331 



Under the epidermis, whose outer walls are much thickened and coated with wax, 

 is the green transpiring tissue or "chlorenchyma", consisting of from five to seven 

 rows of cells. This green tissue does not form a continuous mantle round the stem, 

 but is divided into from ten to fifteen thick strands by strips of hard bast (see 

 ficr. 81). Below this cortex of alternating green tissue and strips of bast are soft 

 bast, cambium, wood, and a very large pith; but these have no further interest for 

 ua here. It is, however, worthy of remark that in the green strands of the cortex 



Fig. 80.— Switch-plants. 

 Bushes of Spartium ncopariuiii near Ruvigno in Istvia. 



of the Spartium, the crowded green chlorophyll-containing cells of the chlorenchj-ma 

 closely adjoin one another, and that only very narrow air-passages ramify between 

 them, so that here there is no formation of a spongy parenchyma penetrated by 

 wide canals and passages. On the other hand, large cavities occur where the green 

 tissue touches the epidermis, and these act as substitutes for the wide ramifying 

 canals.« Over each of the cavities a stoma is to be seen in the epidermis through 

 which the water vapour, exhaled chiefly from the green cells, can escapje (see 

 fig. 81 '-). The stomata are proportionately small, but their number is very great. 

 Since the guard-cells are not so strongly thickened on their outer walls as are the 

 other epidermal cells, the stomata appear to be somewhat sunken. By this arrange- 

 ment, and also by the epidermal coating of wax, they are protected from moisture. 



