STAKCH-GEAINS 695 



cases, again, they are both few and minute, so that they form but 

 a small proportion of the cell-contents. Their nature is at once 

 detected by the addition of a solution of iodine, which gives them a 

 beautiful blue colour. Each granule when highly magnified exhibits 

 a peculiar spot, termed the Jiilmn. round which are seen a set of 

 circular lines that are for the most part concentric (or nearly so) 

 with it. When viewed by polarised light each grain exhibits a dark 

 cross, the point of intersection being at the hiluni (fig. 535) ; and 

 when a selenite plate is interposed the cross becomes beautifully 

 coloured. Opinions have been very much divided regarding the 

 internal structure of the starch-grain, but the doctrine of Nageli 

 that it is composed of successive layers which increase by ' intus- 

 susception/ that is, by the intercalation of fresh molecules of starch 

 between those already in existence, is favoured by many authorities, 

 though the alternative theory of formation by the 'apposition of 

 successive layers also has many advocates. These layers differ in 

 their proportion of water, the outermost layer, which is tin- mo>t 

 solid, having within it a watery layer, this, again, being succeeded 

 by a firm layer, which is followed by a watery layer, and so on. the 

 proportion of water increasing towards the centre in both kinds of 

 layer, and attaining its maximum in the innermost part of the 

 grain, where the formation of new layers takes place, causing the 

 distension of the older ones. Although the dimensions of the 

 Starch-grains produced by any one species of plant are by no means 

 constant, yet there is a certain average for each, from which none 

 of them depart very widely ; and by reference to this average the 

 -t arch-grams of different plants that yield this product in abundance 

 may be microscopically distinguished from one another a circum- 

 stance of considerable importance in commerce. The largest starch- 

 grains in common use are those of the plant (a species of Canna] 

 known as ' tous-les-mois.' The average diameter of those of the 

 potato is about the same as the diameter of the smallest of the 

 ' tous-les-mois,' and the size of the ordinary starch-grains of wheat 

 and of sago is about the same as that of the smallest grains of 

 potato-starch ; whilst the granules of rice-starch are so very minute 

 as to be at once distinguishable from any of the preceding. 



In certain plants, especially those belonging to particular natural 

 orders, the stem, leaves, and other parts are permeated by long 

 branched tubes, constituting the laticiferous tissue. The elements 

 of this tissue may be either greatly enlarged prosenchymatous cells 

 or true vessels. In either case they contain a copious milky-white 

 or coloured juice, the latex, which exudes freely when the part con- 

 taining it is wounded, and dries rapidly on exposure. The chemical 

 composition of the latex varies; it may contain in solution powerful 

 alkaloids, as in the case of the opium-poppy, or gum-resins. Caou- 

 tchouc and gutta-percha are the dried latex of tropical trees and 

 shrubs belonging to several natural orders. Good examples of lati- 

 ciferous tissue are furnished by the Papaveracea?, of which our 

 common field-poppy is an example, many Composite such as ihe 

 dandelion and lettuce, Coiivolvulacea>, Eupho'rbiacese or spurges, 

 Apocynacese, Moracea? including the mulberry &c. 



