TYPICAL VEGETABLE CELL. 51 



blue, proving that they are cellulose in nature. A light line, 

 not blue, but of a yellow color, is in the middle of the cell-walls 

 and marks off the boundary of each cell. This line is the middle 

 lamella. It is composed chiefly of calcium pectate. The cellu- 

 lose portions, as well as the middle lamella, gradually swell, 

 finally dissolve and disappear. 



If a cross-section of the epidermis be treated as above de- 

 scribed the thickened outer wall will be seen to consist of two 

 layers, an inner one, which is cellulose and stains blue, and an 

 outer one, which stains yellowish or brownish. This outer 

 layer is called the cuticle and is composed of a substance known 

 as cutin (cork substance), which does not dissolve in the acid. 

 Cutin is very resistant to reagents and thus forms an excellent 

 protecting layer. The cuticle is represented in Fig. 33. 



The typical cell just described is somewhat advanced from 

 the earliest stage of a cell, known as the primary meristem cell. 

 In such very young cells the wall is ex- 

 ceedingly thin and apparently homo- 

 geneous, the vacuoles are absent and the 

 entire area enclosed by the wall ap- 

 pears to be filled with protoplasm (Fig. 

 34). As the cell grows older its wall 

 becomes thicker and differentiated, as plg 81 _ Very young ce n s . 

 described above. By the expansion of the 



wall the cavity of the cell increases faster than the contained 

 protoplasm, the latter imbibes more water than it is capable 

 of holding in solution, and thus sap cavities or vacuoles are 

 formed, which, at the maturity of the cell, often occupy more 

 space than the protoplasm itself. Finally, when the cell is 

 quite old its living contents disappear' altogether and the cell 

 is dead matter. 



As the cells develop to form the various tissues, a number of 

 substances make their appearance in the contents. Some of 

 these are chlorophyll bodies, aleurone grains, starch, fatty oils 

 and fats, calcium salts, glucosides, alkaloids, sugar, bitter prin- 

 ciples, tannin, resins, gums, inulin, etc. Some of these will be 

 studied in later lessons. 



