6 OF WOOD IN GENERAL. 



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 delicate membranes which 

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

 definite chemical compound known as cellulose. It contains the 

 three elements, carbon, hydrogen, and oxygen, in definite pro- 

 portions, which the chemist represents as C H 10 5 , that is, in 

 a hundred parts by weight 44 are carbon, 6 are hydrogen, and 

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

 group of compounds of carbon with hydrogen and oxygen in the 

 proportions in which those 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, slightly elastic, permeable, but 

 only slightly absorbent, and does not readily undergo fermenta- 

 tion. When treated with acid it passes into the condition of 

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

 under certain conditions in the living 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 

 of some of those substances which, from their resemblance to 



