24 THE GROWTH OF THE TREES. 



This fresh, young growth pushes itself through the open soil 

 and freely imbibes nutrition until in its turn it becomes old 

 and stolid and only of use to produce new shoots. The old 

 roots remain firmly in the ground as they at first grew and do 

 not elongate themselves joint by joint as do the stems. This 

 arrangement is simply a very wise conformance to circum- 

 stances. With ease and freedom the branches and leaves can 

 move in the atmospheric air that enshrouds them ; but it 

 would sadly interfere with the tenacity of the roots' hold on 

 the soil to be continually changing their position. 



As we shall, in this book, confine ourselves to the study of 

 trees and some shrubs, those that have exogenous stems, it 

 would perhaps be well for us to leave for awhile the little plant- 

 let in its upright position with its parts beginning to grow, 

 (Plate II.) and to look further into the material of which it is 

 constructed. The soft tissue alone, while being sufficient for 

 mosses and the lower forms of plant-life, would be too yielding 

 to uphold the weight of foliage that is borne by a tree. At a 

 very early stage, therefore, in large embryos, sometimes even 

 while they are in their seed coat, we find traces of wood-fibre. 

 It occurs also in herbs only in a much smaller proportion than 

 in trees or shrubs. These wood cells, or wood fibre, which we 

 find in the wood that surrounds the central pith are very sim- 

 ilar in construction to those that form the soft tissue ; only 

 they soon lengthen and harden and thicken their walls. Their 

 tapering ends also usually overlap each other in a way that 

 gives to them additional strength. Again in the wood there 

 are ducts : cells which have grown large and long and join to- 

 gether so as to form channels, or tubes that run lengthwise 

 through the wood. They do not thicken their walls. Instead, 

 the so-called dotted ducts are variously marked, sometimes 

 with thin places, like dots and which become .holes as they 

 grow older, while spiral or annual ducts are bound with spirally- 

 coiled fibres, or bands. From the ends of young shoots it is 

 often quite possible to uncoil this filmy thread and in doing so 



