PHYSIOLOGY. 263 



teivdg m different plants, but all terminating in spiral vessels 

 on t,nc cage of the leaf. 



lii heiuaeeous plants, and in young twigs, the epidermis is 

 transparent, receiving its apparent color from that of the cel- 

 lular integument, which it covers. In old trees it appears to 

 be colored, sometimes differently in the same plant. Thus in 

 the White Buch, it is white on the trunk, arid brown on the 

 limbs. 



When this covering is destroyed on the succulent twigs of 

 perennial plants, it is soon renewed, but on the leaf and 

 flower, and in annual plants, it is not reproduced. Mirbel 

 considers the epidermis merely as condensed cellular mem- 

 brane, altered by exposure to air and light, but most other 

 vegetable physiologists consider it as a distinct organized 

 membrane. 



Cutis. This is composed of the epidermis, above described, 

 and a composition of transverse cells, which lie between it 

 and the cellular integuments. These two layers are usually 

 described as a single integument, or membrane, and though 

 they may be separated in many plants, they appear to be de- 

 signed for the same purpose. The Cuticle, or Epidermis in 

 plants, answers to the exteinal membrane of the same name, 

 which covers the entire sunace of animals. In man it is a 

 delicate, insensible covering, designed to protect the nerves 

 and blood vessels of the true skin, from injury. It is that 

 portion of the skin which is raised, and separated in the pro- 

 cess of drawing a blister. See Physiology, 2d Ed. p. 201. 



Cellular Integuments. This portion of the bark is situated 

 immediately under the Cutis, and ib composed of oblong hex- 

 agonal cells, ranging vertically, and varying in regularity in 

 different plants. These cells are usually filled with fluids, 

 which become green by the action of light. The color of the 

 plant, therefore, depends on this part, and is transmitted 

 through the transparent Cuticle, or Epidermis. In herbace- 

 ous plants, growing in the dark, this part is white, and is often 

 seen in the Cabbage, and Potato, which happen to remain in 

 the cellar, until the commencement of the hot season. On 

 exposure of such plants to the sun, they soon become green, 

 in consequence of the production of carbon, by the action of 

 the vessels of this integument. Before such exposure, these 

 plants contained no carbon ; but by the stimulus of the light, 

 the vessels of this part absorb carbonic acid, retain the car- 



