'27(5 VEGETABLE 



takes place through the epidermis, probably in a manner 

 analogous to the process of respiration in animals, where 

 both the oxygenation of the blood, and the moisture thrown 

 out by expiration, is transmitted through the delicate mem- 

 brane of the lungs. (See Respiration, in the author's Phys- 

 iology.) Having undergone the influence of the atmosphere, 

 by which certain chemical changes are produced, the sap re- 

 turns by another set of vessels already described, depositing 

 solid matter in the form of wood and bark, as it descends 

 towards the root. 



VEGETABLE GLANDS. 



The Glands of Plants have already been described at 

 page 51, but perhaps not so minutely as the subject re- 

 quires. 



Internal Glands are generalty seated in the substance of 

 the leaf, with an excretory duct leading to the surface. 

 These furnish the little drops of essential oil which appear 

 on the leaves of some plants, as those of the Black Currant. 

 The odor which many leaves emit, on being slightly rubbed, 

 is caused by the pressure of the oil through the ducts to the 

 surface of the leaf. The Geraniums, Mint, and Bergamot, 

 are well known instances. In most plants when the leaves 

 are swelled by their cellular fluids, these ducts are pressed 

 upon, and closed, and hence the leaf exhales no odor, until 

 it is pressed, or withered. The latter is the case with Cat- 

 mint, the sweet-scented Meadow Grass, and many others. 

 The delightful smell of new hay is caused by this effect in 

 the Meadow Grass. 



External Glands secrete various substances according to 

 the nature of the plants on which they are found. That 

 species of Mesembryanthemum, called Ice-plant, is covered 

 with external glands which produce a gummy substance 

 having the appearance of ice, and hence its name. A sim- 

 ilar secretion is furnished by the glands on the leaves of 

 the Sundew. Other plants, as the pines, secrete a resinous, 

 odoriferous matter, on their small twigs. 



The forms of glands are as various as their produce. 

 With respect to situation, these organs are either sessile, that 

 is, without a foot-stalk, or pedunculated, that is, having a foot- 

 stalk. 



