38 Sfomata. ■ 



sometimes it consists even of several layers of cells much com- 

 pressed, as in some plants of the orchis tribe ; and therefore it 

 would be nnphilosophical to suppose that in different plants the 

 cuticle is made on a different plan ; and, besides, vshen the leaf is 

 young the cuticle is both inseparable and indistinguishable from 

 the cellular tissue of the plant ; and again we may assert that the 

 reticulation of the cuticle being of some figure analogous to that of 

 cellular tissue, it is not altogether incompatible with this idea, as 

 is urged by the supporters of the opposite theory, who give the 

 name of vasa lymphaiica to the veins which traverse it. This 

 question must be allowed to be one of importance, as it is in the 

 cuticle that the whole working of the stomata takes place. 



But to pass on to the stomata or breathing holes which especially 

 distinguish it from any other part of the plant. 



These are composed commonly of two kidney -shaped cells side 

 by side with their extremities generally but not always in contact, 

 with an opening between them, over which they have the power of 

 closing. Occasionally these cells are quadrangular, as in the yucca 

 gloriosa, a Peruvian genus of the liliaceous tribe, and in some the 

 stoma is formed of a greater number than two of these cells, — as in 

 some of the lower tribes, marchantiacese, etc., in which the stomata, 

 being very large, were first observed, and in which they are com- 

 posed of four or five cells. Botanists have disagreed as to the 

 reality of the opening between them, many of the most eminent 

 having entertained an opinion that it is an optical deception. Dr. 

 Lindley, however, seems to have conclusively proved the matter by 

 the following arguments : — that its transjiarency is greater than 

 that of any other part of the cuticle, that it admits transmitted light 

 under the microscope, and that on the light being shifted in various 

 ways, it has shown no signs of any intervening membrane, and 

 when moveable shadows have been thrown between the object and 

 the mirror, it has had no effect on its transparency. 



If then the stoma is a real opening in the cuticle, what is its 

 object? This appears to be connected with respiration, and the 

 fact that stomata have been discovered to be open by day an ' 

 closed by night seem to favour the opinion of the object bein^ 

 exhalation. Especially as it is in those plants that are destitu 

 of transpiration that they are not found. Sometimes probably th 

 serve to facilitate the absorption of gases and perhaps of vapour 

 from the air. They do not, howevei", open actually into cells, but 

 into what are called intercellular spaces, which are generally larger 

 and more numerous on the under side of the leaf than on the 

 upper. That plants do absorb from the air there can be no doubt, 

 and it is reasonable to suppose that the stomata are sometimes used 

 for that purpose. Generally speaking, on examining a piece of the 



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