CONTENTS OF THE TISSUES. 59 



loblast in the manner propounded by Schleiden ; only that the 

 deposit in the case of starch is exogenous, by layer over layer upon 

 a solid nucleus ; while in the cell it is endogenous, or by layer 

 within layer, lining the walls. In both, the solidified matter is in- 

 soluble in cold water ; but in starch it dissolves (or rather swells 

 up into a jelly) and is diffused in boiling water. The deposit on 

 the walls of the cell is of various degrees of density, and some- 

 times exhibits the chemical peculiarity of starch. Though usually 

 permanent, probably it is sometimes redissolved, to be appropri- 

 ated elsewhere. But starch is a temporary formation, for future 

 use ; in which respect it may be compared with the fat of animals. 

 When required for nutrition, the grains are restored to a liquid 

 state in the plant, at the natural temperature ; that is, they are re- 

 converted into Dextrine, a modification of the same substance 

 which is soluble in cold water, and this passes, in' part, at least, 

 into Sugar, which is still more soluble ; and thus a syrup is form- 

 ed, which the sap dilutes and conveys to the adjacent parts wher- 

 ever the process of growth is going on. Physiologically consid- 

 ered, therefore, starch is unappropriated cellulose, stored up in a 

 particular form, as the ready-prepared material of new tissues : 

 while dextrine and sugar are forms in which the same unazotized 

 assimilated matters are prepared for the immediate purposes of 

 nutrition. The part which these substances play in the vegetable 

 economy will be more fully explained elsewhere. 



82. A distinguishing character of starch is that it is turned blue 

 or deep violet by iodine, even in the most dilute solution. Starch- 

 grains are usually simple and separate ; but occasionally two or 

 more young grains join, and are enwrapped by new layers into 

 one. In some plants the grains regularly cohere in united clus- 

 ters. Compound grains of the kind are seen in West Indian Ar- 

 row-root, the corms of Colchicum, Arum,* &c. The starch-grains 

 are nearly uniform in the same plant or organ, and of very differ- 

 ent appearance in different plants : so that the smallest quantity of 

 starch from the potato, wheat, rice, maize, &c., may at once be 

 distinguished under the microscope. 



83. Vegetable Jelly (Bassorin, Salep, Pectine, Vegetable Mucilage 



* The rootstocks of Brasenia and Nymphaea exhibit oblong or club-shaped 

 compound starch grains of great size, very much like those from Arum, rep- 

 resented by Schleiden, on page 17, Engl. Translation. 



