XXIV INTRODUCTION TO BOTANY. 
155. A young and vitally active cell consists of the following parts :—1, 
the outer wall, a permeable, transparent membrane, formed of a chemical 
substance called cellulose; 2, a mucilaginous film lining the wall, and 
called “ the primordial utricle ;”’ 3, the nucleus, or centre of cell-function or 
life, a soft, subgelatinous body occupying the middle of the cell, or ex- 
centrical; and 4, a viscid fluid, called protoplasm, filling the space be- 
tween the nucleus and the primordial utricle. As the cell increases in 
size, its contents change; and finally, when it has attained its proper 
dimensions, the wall formed of cellulose alone remains as a persistent 
fabric, the nucleus is absorbed or dried up, and the protoplasm passes out 
into younger cells. 
156. The principal organized contents of cells are :— 
(a) sap, the first product of the digestion of the inorganic food of 
plants : it contains the elements of vegetable growth in a 
dissolved condition. 
(2) sugar, of which there are two kinds, cane-sugar and grape-sugar, 
usually exists dissolved in the sap. It is found abundantly in 
growing parts, in fruits, and in germinating seeds. 
(ce) dextrine, or vegetable mucilage, a gummy substance intermediate 
between sugar and starch. 
(d) starch or fecula, one of the most universal and conspicuous of 
cell-contents, and often so abundant in farinaceous roots and 
seeds as to fill the cell-cavity. It consists of minute grains, 
called starch-granules, which vary in size and shape, and are 
marked with more or less conspicuous concentric lines. Starch 
is unaffected by cold water, but forms a jelly with boiling water, 
and turns blue when tested by iodine. 
(e) chlorophyll, the green matter of plants, is of a resinous nature, 
and contains nitrogen. It is formed only under the action of 
sunlight, and is usually most abundant in the layers of cells 
immediately below the surface. 
(f) chromule, a name given to a similar colouring-matter when not 
green. 
(g) wax, oils, camphor, and resinous matters are common in cells; 
also various mineral substances, either in an amorphous state 
or as microscopic crystals, when they are called raphides. 
These last are peculiarly abundant in the tissues of the Cacti 
and Rhubarb. 
§ 2. The Epidermis and its Processes. 
157. The Epidermis, or outer skin of plants, is formed of one or more 
layers of vertically flattened, firmly coherent, and usually empty cells, 
with thin and transparent, or with thick and opaque walls. It covers all 
parts exposed to the air, except the stigma and glands; but is absent in 
parts submerged under water. It serves to protect the tissues from the 
immediate action of the air or of drought. 
158. The epidermis is pierced by minute spaces between the cells, called 
stomates. "They are oval or mouth-shaped, bordered by lips, formed of two 
or more elastic cells so disposed as to cause the stomate to open in a moist, 
and to close up in a dry state of the atmosphere. They communicate with 
intercellular cavities, and are obviously designed to regulate evaporation 
and respiration. Stomates are found abundantly on leaves, especially on 
the lower surface ; in succulent leaves, though abundant, they are often in 
an imperfectly organized condition, and seemingly inactive. 
159. Hairs ave minute, transparent expansions of cellular tissue, pro- 
ceeding from the epidermis. The hairs of roots are active absorbents ; 
