xvi 
permanent, more or less woody trunk. In exogenous trees, veve- 
tation is chiefly carried on through the inner bark and young 
wood, the heart-wood and outer bark retaining little vitality, as 
may be often seen in the case of hollow trees, which, though the 
trunk and principal branches are reduced to a mere shell, the inte- 
rior having wholly decayed, still continue to put forth leaves and 
flowers for many successive years. 
All the organs of plants consist of cells of various form and 
arrangement, either connected in masses, forming what is called 
cellular tissue, or elongated and joined together end to end so as to 
present a tubular appearance. Woody fibre is made up of a num- 
ber of such tubular cells, united by their overlapping extremities, 
These cells are all originally membranous ; and communication is 
established between the liquids contained within them by the pecu- 
liar process of exudation called endosmose. During growth, how- 
ever, the walls of the cells are thickened by the gradual deposition 
of earthy and other substances from the liquid, and are sometimes 
converted in this manner into a solid mass. Growth is the forma- 
_ tion of new cells, either by a process of subdivision and extension 
of those previously existing, or from the development of a minute 
body called a nucleus, which, formed in the cell-fluid, is afterwards 
extended through the membrane and becomes a cell itself. In each 
_ cell, during growth, a fluid is constantly circulating with more or 
less rapidity, holding in suspension various matters which are 
eventually deposited within it. One of the most important of these 
organic matters is the chlorophyll or colouring-substance of the 
plant, usually of a ereen tint. Light is necessary for its produc- 
tion ; and consequently, when plants are grown in the dark, they 
become etiolated or blanched. Starch, gum, and sugar form the 
principal other substances floating in the cell-fluid ; they are pre- 
sent in all plants during growth. Besides these, various resinous 
65 oily secretions are elaborated in certain cells of the plant, prin- 
cipally in those containing that portion of the fluid which has 
OO EE 
