SECRETION. 583 



ganic salts or compounds of organic acids with lime &c., are frequently 

 met with in the cellular tissues j but very little is known at present of 

 the nature of their relation to the chemical processes of vegetation. 

 Calcium oxalate may be regarded, according to Holzner and Hilgers, as a 

 product of elimination, by means of which the superfluous lime is depo- 

 sited in a solid form. This lime is set at liberty by the calcium phosphate 

 absorbed, when this is decomposed to supply the phosphoric acid requisite 

 for the new albuminoid materials. The lime so liberated is combined 

 with the oxalic acid with which young tissues abound. Rauwenhoif 

 shows that these raphides are wanting in plants grown in obscurity; 

 which, therefore, is an indication of inability of the plant to absorb or 

 decompose calcic phosphate. 



The close relation of the vegetable acids, oxalic, malic, citric, &c., to 

 carbonic dioxide, water, and the ternary assimilated substances has 

 already been alluded to. 



Tannin, or tannic acid, is a very frequent constituent of the woody 

 tissues when their vital activity has ceased, and is perhaps a product of 

 decomposition. It is formed in plants exposed to li^ht as well as in 

 those grown in obscurity, but in smaller proportions in the latter case. 

 Oak, Sumach, Ithus coriaria, Acacia catechu, &c. owe their tanning pro- 

 perties to this substance. In fruits the proportion of tannin decreases as 

 that of sugar increases, but it is not known what circumstances regulate 

 the change. For the detection of tannin in the cells under the micro- 

 scope, Sanio recommends that sections be macerated in potassic bichro- 

 mate, which causes a reddish-brown precipitate in the cells, and which 

 is consistent enough to allow of sections being made without extravasa- 

 tion and staining of adjacent tissues. 



Speaking in general terms, it may be said that the immediate 

 principles of plants may be grouped under three heads: 1, those 

 in which oxygen is in excess as pectin, pectose, tannin, and vege- 

 table acids ; 2, those in which H and O are in equal proportions 

 cellulose, starch, gum, sugar, lactic, acetic, quinic acids ; and, 3, 

 those where H is in excess as in resins, essential oils, camphor, 

 salicin, various alkaloids and colouring-matters. 



