2 ELEMENTAET OEGANS. 



it is Cj Hi„ O5.* It was long considered as essentially a vegetable 

 product, not found in animal structures ; but it has now been de- 

 tected in the tissues of the ascidia, and other molluscous animals. 

 It is a white substance, insoluble in water, alcohol, or ether, 

 but soluble ia an ammoniacal solution of cupric oxide. It is aUied 

 to starch, into which it is convertible by the action of heat, the 

 addition of sulphuric acid, or caustic potash. It becomes yeUow on 

 the addition of iodine, and when acted upon by iodine and sidphuric 

 acid, a blue colour, like that of iodide of starch, is produced. The 

 acid appears to convert the cellulose into starch. When cellulose is 

 acted on by a mixture of equal volumes of strong sulphuric and nitric 

 acid it forms gun-cotton (pyroxylin), (tDj, fire, and f uXok, wood), and 

 this when dissolved in a mixture of ether and alcohol yields a solution 

 called collodion. The membrane formed by ceUulose is permeable by 

 fluids, and becomes altered in the progress of growth, so as to acquire 

 various degrees of consistence. A modification of cellulose occurs in 

 the form of woody matter or lignin. The hard cells in the stone of 

 the peach, in the shells of other fruits, and in the coats of seeds, 

 consist of cellulose, with deposits of lignin. In the advanced stages 

 of growth, plants consist of two 'kinds of tissue, Cellular and Vascular, 

 which, under various modifications, constitute their Elementary organs ; 

 and these, by their union, form the Compound organs, by which the 

 different functions of plants are carried on. 



The elementary organs are vesicles and tubes, which vary in form 

 and size, and, when united in different ways, constitute the tissues. 

 Vesicles or cells may be defined as closed sacs, composed of 

 r% solid membrane, containing fluid or semifluid matter, and 

 ^^ having a diameter nearly equal in every direction (fig. 1); 

 Fig- 1- while tubes or vessels are similar sacs with the longitudinal 

 much exceeding the transverse diameter (figs. 3, 4). Cellular tissue 

 is formed by a combination of these cells or vesicles ; a similar union 

 of vessels constitutes vascular tissue. 



Fig. 1. Vesicles or small cells, each of them enclosed hy a membrane of cellulose. 



* These symbols indicate the equivalents of Carbon (C), Hydrogen (H), and Oxygen (O), 

 which enter into the composition of cellulose. For the meaning of these and other chemical 

 symbols, see Chap. II. Sect. I. Div. 2, on the Food of Plants. 



