Ctiltural First Principles. 533 



liquid substance, the cell- sap. Cells seldom, however, occur in their 

 elementary spherical form. More generally they are more or less 

 compressed and adhere together in what are known as vegetable 

 tissues. 



Of the vegetable tissues, the most important to the forester is un- 

 doubtedly wood, which owes ita importance to the length of its con- 

 stituent cells, their intertwining, and their hardness from the substances 

 deposited on their interiors. 



The cells in wood are mostly elongated, tapering at each end, and 

 adhering by their sides. As they grow older cells often, especially in 

 wood, deposit cellulose, and another carbohydrate, known as lignine, 

 on the inside of their cell walls, and great changes occur in the con- 

 tents of the cell, for the cell- wall is readily permeable to liquids. 

 Cells vary in size, the spores of some fungi being less than Yrm 

 of an inch in diameter, while in the pulp of a lemon many cells are 

 more than half an inch longj, and the hairs on the seeds of a shrub 

 known as Gossypium, which are familiar to us as cotton, are single 

 cells often two inches long. 



Plants take in liquid and gaseous food through their cell-walls, and 

 the protoplasm of growing cells is capable of dividing and secreting 

 a partition of cellulose, so as to form two cells, or even four, out 

 of one. 



The growth of a plant is then nothing more than the aggregate 

 result of the enlargement and multiplication by cell division of the 

 cells which compose it. 



Some vegetable cells grow and multiply with astonishing rapidity. 

 Thus a puff-ball has grown to a foot in diameter in a single night ; 

 this representing the production perhaps of three or four hundred 

 million cells per hour, though it may have been partly due to the en- 

 largement of previously formed cells. 



In the centre or heartwood of timber trees the cells are physiolo- 

 gically dead, the vital fluids having ceased to circulate in them, owing 

 to their being blocked up by the formation of lignine. This is what 

 renders this dead tissue valuable as timber. Again, in the bark of 

 trees there are cells containing nothing but air. Of such cells is cork 

 composed — the bark of Quercus suhcr — and from this contained air it 

 derives its utility. 



Attention to some simple principles such as these might save many 

 a misconception, such, for instance, as the statement which circulated 

 from America, through many of our " scientific " papers, that diatoms 

 had been found in the cells of wheat-straw. The various kinds of 

 tissues are of importance to the botanist, but the forester need not 

 carry his studies into such detail, but may at once go on to learn how 

 the tissues build up the different organs of the plant. 

 VOL. I. 2 p 



