56 THE ELEMENTARY STRUCTURE OF PLANTS. 



by its centre to the epidermis. They are well shown in the Ole- 

 aster, Shepherdia, and most silvery leaves like theirs. Our spe- 

 cies of Vesicaria exhibit beautiful gradations between these and 

 stellate hairs. 



SECT. IV. OF THE CONTENTS OF THE TISSUES. 



77. These comprise all the products of plants, and the materials 

 they take in from which these products are elaborated. To treat 

 of them fully would anticipate the topics which belong to the 

 chapter on nutrition. Some of the contents of cells, however, 

 have already been mentioned, in the account of their production 

 and growth (27-39): others require a brief notice here, espe- 

 cially two solid products which are of nearly universal occurrence 

 and great importance in the vegetable economy, namely, Chloro- 

 phyll and Starch ; and a third, which, however constant, may be 

 regarded as a kind of accidental deposit, namely, Raphides or 

 Crystals. 



78. The same cells contain liquids, solids, and air, at different 

 ages. Growing and vitally active cells are filled with liquid (at 

 least while vital operations are carried on), namely, with water 

 charged more or less with nutritive assimilated matters, the pre- 

 pared materials of growth (11, 27). Any gaseous matter they 

 may contain at this period is, for the most part, held in solution. 

 Completed cells may still be filled with liquid, or with air or solid 

 matter only. The liquid contents of the vegetable tissues, of what- 

 ever nature or complexity, are often spoken of under the common 

 name of 



79. Sap, In employing this name we must distinguish, first, 

 CRUDE SAP ; the liquid which is imbibed by the roots and carried 

 upwards through the stem. This is water, impregnated with cer- 

 tain gaseous matters derived from the air, and with a minute por- 

 tion of earthy matter dissolved from the soil. It is therefore inor- 

 ganic (12). But, as it enters the roots and traverses the cells 

 in its ascent, it mingles and necessarily becomes impregnated 

 with the liquid or soluble assimilated matters which these contain 

 (37). On reaching the leaves, the inorganic materials are trans- 

 formed, under the influence of light, into organizable or assimilat- 

 ed matter ; and the liquid, thus charged with the ready prepared 

 materials of growth, is now ELABORATED SAP. The two classes of 



