86 Vegetable Physiology. 



face of the water, and those which grow partly in the water pos- 

 sess it only on those parts which are not submerged. When it 

 is destroyed on the succulent twigs of perennial plants it i s 

 soon renewed, but on the leaf and flower, and on annual plants 

 it is not reproduced, and the part from which it is removed soon 

 withers. The cuticle possesses certain peculiar apertures 

 called stomata, from the Greek, signifying mouths. They are 

 usually of an oval form, but sometimes square, or circular, and 

 are bounded by two curved or kidney-shaped vesicles, by the 

 contraction or expansion of which the orifice is diminished or 

 increased. The appearance and structure of some of these 

 is shown in Figure 7, a and b. 



Fig. 7. 



Stomata are chiefly found on the soft green tissue of young 

 shoots and leaves, and on such succulent stems as those of the 

 Cactus or Prickly Pear, and they always open into intercellu- 

 lar passages. On the upper surfaces of many leaves whose 

 tissue has become hardened, they seldom occur: but in other 

 leaves, particularly those which grow upright, as those of the 

 Iris and Flag, they are common to both sides. No traces of 

 them are seen in the sea weeds, and few in the mushrooms or 

 lichens ; but in the Liverwort tribe they occur in a remarkably 

 complex form, and in all flowering plants they abound. Their 

 size is very minute, being only from the 3000th to the 1500th 

 of an inch in length. Their office seems to be connected with 

 the evaporation of water from the inner tissue, which is one of the 

 processes by which the fluid absorbed by the roots is converted 

 into the nutritious sap, or proper juice. Some curious experiments 

 have been made to show the effect of light in the production of 

 stomata. There is a common kind of Liverwort, called Mar- 

 chantia polymorpha, which grows in moist situations, and has a 

 very peculiar system of fructification. It produces small leafy 

 buds, which spontaneously separate themselves from the parent 

 structure, % and become new plants. In these buds no stomata 

 or roots exist when first separated, but as soon as they begin 



