140 PRODUCTS AND SECRETIONS OF PLANTS. 



watered during the voyage, and received no protection by day or by 

 night, but were taken out at Loddiges in a most healthy and vigorous 

 condition. 



299. It is a mistake to suppose that the air in the Cases is not changed. 

 They are not henneticaly sealed; and by the law of diffusion of gases 

 there is a constant although gradual mixture of the external air, free 

 however from many impurities, with that inside. Plants will con- 

 tinue to grow for a long time, even in Cases hermetically sealed, if 

 supplied at first with abundance of good soil and water. By the united 

 action of the plant and light, the air undergoes constant changes, and 

 thus continues fit for vegetable life. 



4. PRODUCTS AND SECRETIONS or PLANTS. 



300. The sap, in its progress through the cells and vessels, and espe- 

 cially in its passage through the leaves, is converted into organizable 

 products, from which the vegetable tissues and the secretions contained 

 in them are ekborated. Light, by enabling plants to fix carbon, has 

 an important influence over these secretions. When plants are kept 

 in darkness they become etiolated or blanched, and do not form their 

 proper secretions. Gardeners resort to the practice of blanching when 

 they wish to diminish or destroy certain secretions, and to render 

 plants fit for food. In speaking of the contents of cells and vessels, 

 allusion has already been made to some of the more important 

 organizable products. It is proposed in this place to take a general 

 view of those vegetable secretions which are connected with the 

 nutrition of plants, or which are important on account of their medical 

 or commercial uses. Some of these occur in small quantity, and 

 are limited to certain plants only; others are abundant, and more 

 universal in their distribution. Thus, while quinine and morphine, the 

 active ingredients of Peruvian bark and opium, are circumscribed, 

 both as regards quantity and distribution, starch, gum, sugar, woody 

 matter, and certain nitrogenous compounds are more abundant, and 

 more generally diffused over the vegetable kingdom. The latter 

 substances therefore demand especial attention. If a plant is macer- 

 ated in water, and all its soluble parts removed, lignine or woody 

 fibre is left, and the water in which it has been macerated, gradually 

 deposits starch. If the liquid is boiled, a scum coagulates, formed of 

 albumen and some azotised matters, while gum and sugar remain in 

 solution. 



301. Starch is a general product, being laid up as a store of nourish- 

 ment, and undergoing changes at certain periods of a plant's life, which 

 fit it for further uses in the economy of vegetation. It is not found in 

 animal cells. It consists of C 12 H 10 O 10 , and occurs in the form of 

 grains of various sizes and forms, having an external membrane, en- 



