So 



the palisade layer, contain chlorophyll. They form the lower 

 and larger half of the leaf substance. 



The stomata which we have mentioned communicate directly 

 with the intercellular spaces which are nearest the lower surface. 

 We shall presently see the functions of all these parts, in con- 

 nection with the upward movement of fluid in the stem : but 

 first a few words about Transpiration. 



Every portion of a plant which is exposed to the atmosphere, 

 and which is not covered by thick layers of cork, e. g. the stem, 

 or by cuticle, e. g the leaves, both of which are equally imper- 

 vious to air or water, is constantly giving off enormous quanti- 

 ties of water) vapour into the air by evaporation. This exha- 

 lation or loss of watery vapour from any part of a plant is 

 known as Transpiration, and it takes place, as I have said, 

 wherever a cell is exposed to the air. Water- weeds for example, 

 such as P t nogeton, when dragged out on the bank, very 

 quickly shrivel up, and lose their water faster than aerial 

 plants do : this is due to the fact that the epidermis is not cuti- 

 culanzed in them, and consequently transpiration takes place 

 from the entire surface of the plant. Owing, however, to the 

 principal parts of plants being covered either by cuticle or by cork, 

 it is carried on for the most part by means of certain openings, 

 viz., the stomata of which we have just spoken; these stomata 

 occur on all parts of the more highly organized plants which 

 are above the ground, viz. , on stems, leaves, and the petals of 

 flowers ; they extend also very far down in the vegetable king- 

 dom, thus we find them among the Mosses and Liverworts 

 {Hepaticae). Below this last named group we find no trace of 

 them, but there is no need for them, since all the cells are more 

 or less in contact with the air or water, and the exchange of 

 gases can take place through their walls. 



The leaves are the principal organs of transpiration, because 

 in them, as we have seen, the stomata open into the inter- 

 cellular spaces, which are bounded by the cell -walls of many 

 cells. Transpiration does occur also, however, in stems pro- 

 vided with stomata and epidermis (herbaceous plants), or even 

 where the epidermis has become transformed into cuticle, if the 



