STRUCTURE AND SITUATION OF THE STOMATA. 73 



or contraction of these, that the orifice is diminished or increased. 

 Sometimes, however, the opening is round, and is bounded by a 

 ring of four or five such cells; and in the very curious stomata of 

 the Marckantia potymorpha^ one of the commonest of the Liver- 

 wort tribe (. 32), there are five such rings, one beneath the 

 other, the aperture resembling a funnel, and the lowest ring being 

 the one, which regulates the amount of communication, between 

 the chamber into which it opens, and the external air. 



Fro. 32. VIEWS OF STOMATA. A, vertical section of stoma of Iris ; a, a, green cells 

 bounding the orifice ; b, b, cells of the parenchyma ; c, air chamber. B, view of the 

 same from above ; a, a, green cells of the stoma, lying between long cells of the cuticle ; 

 c, opening between them. C, similar view of a stoma of apple leaf ; a, cells of the 

 stoma ; 6, 6, cells of the cuticle ; c, opening of stoma. 



92. Stomata are always placed over interspaces in the tissue, 

 which are called intercellular passages ; they are never found on 

 the midrib or veins of a leaf, nor in fact over any hard woody 

 portion of the structure. They are chiefly disposed over the soft 

 green tissue of leaves and young shoots ; but they are found also 

 on the parts of the flower. "When the leaves are absent, and the 

 stem performs their functions, as in the Cactus or Prickly-pear 

 tribe, stomata are found on its surface. They are generally 

 most abundant on the under surface of leaves, and are sometimes 

 altogether absent from the upper. This is partly due to the fact, 

 that the tissue lying beneath the upper surface of leaves is so 

 closely packed together, that there are scarcely any intercellular 

 passages, into which the stomata might open ; whilst the tissue in 

 contact with the lower cuticle is extremely loose in comparison, 

 and abounds with such passages ; hence it is, that the colour of the 

 upper surface of the leaf is usually so much deeper, than that of 

 the lower. But in leaves of which the two sides are equally ex- 

 posed to the air and light, such as those of the Iris, and of the 

 common Flags growing by the sides of brooks, the general struc- 



