162 YEGETABLE PRODUCTS — STARCH. 



secretions are elaborated. 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 ; a familiar example of which may be 

 seen in their culture of Apium graveolens (Celery). In speaking of 

 the contents of cells and vessels, allusion has already been made to 

 some of the more important organisable 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 medicinal 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 

 quinia and morphia, the active .ingredients respectively 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 difiused through- 

 out the vegetable kingdom. The latter substances therefore demand 

 special 'attention. If a plant is macerated in water and all its soluble 

 parts removed, lignin is left, and the water in which it has been 

 macerated gradually deposits starch. If the liquid is boiled a scum 

 coagulates, formed of albumin and some azotised matters, while gum 

 and sugar remain in solution. 



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 

 usually found in animal cells. It consists of Og Hj,, Og, and occurs 

 in grains of various sizes and shapes, having an external membrane, 

 enclosing a soluble substance. By boiling in water, the pellicle bursts, 

 and the contents are dissolved, becoming gelatinous on cooling. The 

 circular markings and striae seen on the grains, and the part called the 

 hilum, have already been noticed (p. 10). The grains of potato starch, 

 seen by polarised light, exhibit a well-marked black cross, the centre 

 of which corresponds with the hilum. Some plants, such as potato, 

 arrow-root, and wheat, contain a large quantity of starch, which varies, 

 however, in quantity according to the period of growth. Thus, while 

 starch abounds in the potato towards the latter part of the season, it 

 decreases when the tubers begin to germinate in spring. It was found 

 that 240 lbs. of potatoes, left in the ground, contained of starch — 



