XXVI 
OUTLINES OF BOTANY. 
the principal part of wood, of the inner bark, and of the nerves and veins of leaves, 
forming, in short, the framework of the plant. 
(3) Vascular tissue, or the vessels or ducts of plants, so called from the mistaken no- 
tion that their functions are analogous to those of the vessels (veins and arteries) of 
animals. A vessel in plants consists of a vertical row of cells, which have their trans- 
verse partition-walls obliterated, so as to form a continuous tube. All phsenogamous 
plants, as well as ferns and a few other cryptogamous plants, have vessels, and are 
therefore called vascular plants ; so the majority of cryptogams having only cellular 
tissue are termed cellular plants. Vessels have their sides very variously marked; 
some, called spiral vessels, have a spiral fibre coiled up their inside, which unrolls 
when the vessel is broken ; others are marked with longitudinal slits, cross bars, 
minute dots or pits, or with transverse rings. The size of vessels is also very variable 
in different plants ; in some they are of considerable size and visible to the naked eye in 
cross sections of the stem, in others they are almost absent or can only be traced under 
a strong magnifier. 
189. Various modifications of the above tissues are distinguished by vegetable ana- 
tomists under names which need not be enumerated here as not being in general prac- 
tical use. Air-vessels, cysts, turpentine-vessels, oil-reservoirs, etc., are either cavities 
left between the cells, or large cells filled with peculiar secretions. 
190. When tissues are once formed, they increase, not by the general enlargement of 
the whole of the cells already formed, but by cell-division, that is, by the division of 
young and vitally active cells, and the enlargement of their portions. In the formation 
of the embryo, the first cell of the new plant is formed, not by division, but around a 
segregate portion of the contents of a previously existing cell, the embryo-sac. Tliis is 
termedyVee cell-formation, in contradistinction to cell-division. 
191. A young and vitally active cell consists of the outer wall, formed of a more or 
less transparent substance called cellulose, permeable by fluids, and of ternary chemical 
composition (carbon, hydrogen, and oxygen) ; and of the cell-contents, usually viscid 
or mucilaginous, consisting of protoplasm, a substance of quaternary chemical compo- 
sition (carbon, hydrogen, oxygen, and nitrogen), which fills an important part in cell- 
division and growth. Within the cell (either in the centre or excentrical) is usually a 
minute, soft, subgelatinous body called the nucleus, whose functions appear to be inti- 
mately connected with the first formation of the new cell. A 3 this cell increases in size, 
and its walls in thickness, the protoplasm and watery cell-sap become absorbed or dried 
tip, the firm cellulose wall alone remaining as a permanent fabric, either empty or filled 
with various organized substances produced or secreted within it. 
192. The principal organized contents of cells are 
sap, the first product of the digestion of the food of plants ; it contains the ele- 
ments of vegetable growth in a dissolved condition. 
sugar, of which there are two kinds, called cane-sugar and grape-sugar. It usually 
exists dissolved in the sap. It is found abundantly in growing parts, in fruits, and in 
germinating seeds. 
dextrine, or vegetable mucilage, a gummy substance, between mucilage and starch. 
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 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. 
chlorophyll, very minute granules, containing nitrogen, and coloured green under 
the action of sunlight. These granules are most abundant in the layers of cells imme- 
diately below the surface or epidermis of leaves and young bark. The green colouring 
matter is soluble in alcohol, and may thus be removed from the granules. 
chromule, a name given to a similar colouring matter when not green. 
wax, oils, camphor, and resinous matter, are common in cells or in cavities in the 
tissues between the cells, also various mineral substances, either in an amorphous state 
or as microscopic crystals, when they are called Raphides. 
