Physiological Points 17 



conditions. To this \'ery important relation which exists 

 between root and crown development we shall refer again 

 further on. 



Instead of leaves, the youngest parts, i.e., the tips of the 

 roots, are beset with fine hairs; delicate structures which 

 serve the purpose of taking up water. The root-hairs live 

 only a very short time, hardly through the season, being 

 ever replaced by new hairs near the tip, which constantly 

 pushes into new soil. The tips of the roots, therefore, are 

 the most important part of this organ for the Hfe functions 

 of the tree. 



Physiological Points. We may now take a brief glimpse 

 into the household atTairs of the tree, — the manner in which 

 it lives. 



The essential fact which must be realized is, that the 

 leaves and buds at one end, and the tips and youngest parts 

 of the roots at the other end, with the cambium layer con- 

 necting the two, are the living tissues, and hence the main 

 factors in the household. The tips and youngest ends of 

 the roots or rootlets, densely beset with little hairs, are the 

 mechanism which takes up the water and minerals from 

 the soil. These are conducted through the roots by way 

 of the cambium and the younger wood and bark to the 

 leaves. The leaves in turn take up carbonic acid from the 

 air, decompose it in their green parts (chlorophyll) under 

 the influence of light, and combine it with the minerals and 

 w'ater into food materials which can be used in building up 

 the body of the tree. This process is called assimilation. 

 The food materials, manufactured in the lea\es, wander 

 through the veins and stalks of the leaves into the cambium 

 and the youngest wood and bark of the stem and root, 

 where they are used in new growth during the season, making 

 the annual ring, or else are stored (especially in the pith rays) 



