USE OF WOOD TO THE TREE 5 



dissolved gases and saline substances, taken in by the root from 

 the soil, are conveyed to the leaves, which have been termed the 

 *' laboratory of the plant," to be built up in them, "with the 

 carbonaceous food-material taken in from the atmosphere, into 

 those complex " organic " compounds of which the whole struc- 

 ture of the plant is composed. Furthermore, the stem serves 

 as a reservoir in which some of these organic compoundSj the 

 " plastic material " of the plant, are stored up for use in future 

 growth. 



Every stem and every branch — and a branch is but a secondary 

 stem, differing only in position — as long as it remains capable of 

 elongation, is terminated, in the groups of trees with which we 

 are concerned, by a bud. A bud is a growing-point protected by 

 overlapping rudimentary leaves. 



In the immediate neighbourhood of this growing-point the 

 stem in this its initial stage is entirely made up of structures 

 which almost completely resemble one another. Whether we 

 cut such a growing-point across or lengthwise it presents under 

 the microscope the appearance of a delicate mesh-work of thin 

 membrane filled in with a viscid semi-fluid substance. These 

 meshes, from their resemblance to honeycomb, were in 1667 

 named cells by Robert Hooke. The dehcate membranes which 

 form them, the cell-walls as they are termed, are composed of a 

 substance, or rather group of substances, known as cellulose. It 

 contains the three elements, carbon, hydrogen, and oxygen, in 

 definite proportions, which the chemist represents as CgH^^QOg, that 

 is, in a hundred parts by weight 44 are carbon, 6 are hydrogen, 

 and 60 are oxygen. Cellulose, like starch and sugar, belongs to a 

 group of compounds of carbon with hydrogen and oxygen in the 

 proportions in which these two elements occur in water, which 

 are known as carbo-hydrates. It has, in fact, the same percentage 

 composition as starch, though differing from it in many properties. 

 It is insoluble in water, flexible, sHghtly elastic, permeable, but 

 only slightly absorbent, and does not readily undergo fermenta- 

 tion. When treated with acid it passes into a starch-like condi- 

 tion, as is evidenced by its then turning blue with iodine, and 

 under certain conditions in the Hving plant it would seem capable 

 of being formed from, or of passing into, sugar. Cotton-wool 

 consists almost entirely of pure, unaltered cellulose. The viscid, 

 semi-fluid substance contained in the cells is of far more complex 

 chemical composition. It contains not only carbon, hydrogen, 

 and oxygen, but also, though in far smaller proportion, nitrogen, 

 with traces of sulphur, and, perhaps always also, phosphorus and 

 other elements. It is probably a mixture in varying proportions 



