USE OF WOOD TO THE TREE. 5 



gennao, to produce. The term enddgenous, still sometimes applied 

 to the structure of the stem of monocotyledons, is less accurate. 

 Dicotyledons are commonly slower of growth than conifers, and 

 their wood, especially that near the centre of the stem, is often 

 much harder. They bear as a rule also broad, net-veined leaves ; 

 and are known familiarly, therefore, as "hardwoods" or as 

 " broad-leaved trees." Such are the Oak, Beech, Ash, Elm, Teak, 

 Willow, Alder, etc. 



It is then only with the two classes of exogenous stems, those 

 of gymnosperms or needle-leaved trees, and those of dicotyledons 

 or broad-leaved trees, that we are concerned. 



Though, as we have already said, conifers and broad-leaved 

 trees present important differences in the structure and conse- 

 quent character of their wood, their manner of growth is so 

 nearly identical in its initial stages and broad outlines that 

 we may well treat them at first collectively. It is, perhaps, 

 the many branches and the numerous small leaves exposed by 

 means of those branches to a maximum of air and light in 

 these two groups of plants (as contrasted with the general 

 absence of branching, and the small number and large size of 

 the leaves in ferns and palms) that has determined the produc- 

 tion of the progressively-enlarging, solid stem that characterizes 

 them. It must be remembered, however, that the stem of a 

 tree fulfils several very distinct physiological purposes. Besides 

 bearing up the weight of leaves and flowers so as best to obtain 

 the air and light they require, it is the means of communication 

 between the root and the leaves. Through it the water and its 

 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 compounds, the 

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

 growth. 



