xX INTRODUCTION TO BOTANY. 
154. Various modifications of cellular, woody, and vascular tissues are distin- 
guished by vegetable anatomists, but need not be here enumerated. Other false- 
elemen organs, such as air-vessels, cysts, turpentine-rgssels, oil-reservoirs, &c., are 
all either intercellular cavities, or large cells filled with peculiar secretions. 
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,” 
8, the nucleus, or centre of cell-function or life, a soft, sub-gelatinous body occupying 
ing the middle of the cell, or excentrical ; and 4, a viscid fluid, called protoplasm, 
filling the space between the nucleus and the primordial utricle. As the cell in- 
creases in size, its contents change ; and finally, when it has attained its proper di- 
mensions, 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. 
(4) 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. 
(c) dextrine, or vegetable mucilage, a gummy substance intermediate between 
sugar and starch. 
vary in size and shape, and are marked with more or less conspicuous 
concentric lines of growth. The chemical constitution of starch is the 
~ same as that of cellulose ; it is unaffected by cold water, but forms a 
jelly with boiling water, and turns blue when tested by iodine; when 
fully dissolved it is no longer starch, but dextrine, 
(e) —— a substance intermediate between starch, dextrine, and cellu- 
ff) chlorophyll, the green matter of plants, is of a resinous nature, and con- 
tains nitrogen. It is formed only under the action of sunlight, and is 
a most abundant in the layers of cells immediately below the sur- 
(g) chromue, a name given to a similar colouring matter when not green. 
(A) wax, oils, camphor, and resinous matters are common in : also 
various mineral substances, either in an amorphous state, or as micro- 
scopic crystals, when they are called raphides. These last are peculiarly 
abundant in the tissues of the Cacti and rhubarb. a 
§ 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 transpa- 
rent, 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 
