STRUCTURE OP STOMATBS. 17 



mid-rib and leaf-stalk, and go to form the twig, branch and stem, or 

 triink and root. 



When the leaf itself is broken across, or cut, it may appear to be as 

 homogeneous as a sheet of paper of the same thickness, with scarcely 

 space sufficient for any more complicated structures; but when 

 examined with a microscope of considerable power it is seen to be 

 composed of a great number of cells piled loosely together, six or 

 eight or even more it may be in depth, with large spaces 

 intervening, and framed by a layer of similar cells, compactly 

 arranged like a brick floor, forming the back of the leaf, 

 and a double or triple layer of cells, similarly compactly arranged 

 like brick-work, two bricks or three bricks thick, forming the face of 

 the leaf. These are compact enough, but those constituting the 

 body of the leaf, filled with sap, are so loosely grouped, apparently 

 without any very determined or strongly marked arrangement, that 

 they only touch each other on parts of their surface, and the air may 

 play freely almost entirely round each and through amongst them 

 all. A simpler arrangement for thorough ventilation perhaps man 

 could scarcely imagine. 



In the back of the leaf are numerous stomates, or mouths. The 

 structure of these differs in different plants, but what may be con- 

 sidered the typical structure is two elongated cells, resembling a 

 microscopic black pudding or thick sausage, so built into the struc- 

 ture of the skin of the leaf that this will not admit of their being 

 further elongated ; each of these is, along one side, attached to that 

 skin, but on the sides along which they are in contact they are free. 

 When moisture is in excess they become distended, but the structure 

 of the skin of the leaves is such that they cannot be elongated, and 

 they bulge away from each other, leaving a wide opening between 

 them through which the vapour with which the air surrounding the 

 cells in the interior of the leaf is charged finds an open exit. When 

 the pressure is relieved they, having lost some of the moisture or water 

 with which they were filled and distended, collapse to such an extent 

 as to diminish the opening ; and in this way, exactly to the degree 

 required, they vary and regulate that aperture — varying it, it may be, 

 I shall not say twenty times in the day, but, if necessary, twenty 

 times in the minute ; and if drought become such as to render it 

 desirable that every drop of moisture in the plant should be preserved, 

 under the influence of that drought they become flaccid and com- 

 pletely close the aperture. 



Such is the arrangement, in these galleries of the laboratoj-ies of the 



