2 % J6 GENERAL CHARACTERS OF VEGETABLE SECRETIONS. 



sent out of the body for particular objects. But Secretion in 

 animals has other purposes ; namely, to purify the blood from 

 certain ingredients, which, if they accumulated in it, would 

 occasion disease and even death. This is the purpose of the 

 separation of carbonic acid by the lungs ; and also, in part, of the 

 secretion of bile, which carries off a large quantity of the super- 

 fluous carbon of the system. In the same manner, the secretion 

 of urine carries out the superfluous nitrogen, which exists very 

 largely in this fluid. (See ANIM. PHYSIOL. Chap, vn.) 



361. Now in regard to the Secretions of Plants, it is very 

 remarkable that, whilst in number and variety they much exceed 

 those of Animals, the use of them in the Vegetable economy 

 should be much more obscure. In a few instances only are they 

 destined to be sent out of the system ; they are usually deposited 

 in some part of it ; yet they are not even separated in every in- 

 stance, from the nutritious part of the juices, in which they are 

 at first formed. The Secretions of plants comprehend all the 

 peculiar products, which do not form part of their tissues ; thus, 

 all the vegetable dyes, the active medicinal principles, the oils, 

 resins, &c., and the aromatic or volatile oils, belong to this class 

 of products. Now as the substance of which the tissues of 

 plants are composed, is everywhere almost the same, any varieties 

 which these tissues may present, in colour, taste, &c., must be 

 due to them ; and it is from their presence, that each plant 

 derives its particular character, either as an article of food, or as 

 furnishing products useful in medicine or the arts. The pure 

 vegetable tissue, and the nutritious gum or starch combined with 

 it, are nearly tasteless ; and the alburnum or sap-wood of trees 

 possesses neither toughness nor colour. The former may be 

 rendered uneatable, by the disagreeable taste or injurious nature 

 of the secretions diffused through it ; the latter is strengthened, 

 and receives its peculiar colour, by the deposition in its cells and 

 tubes, of products which have been separated from the circulating 

 fluid, and which give to the wood a density proportionate to 

 their amount, and to their own power of subsequently hardening. 



362. The formation of these Secretions is still more dependent 

 on the influence of light ^ than is that of the nutritive materials 



