TREE-GROWTH. 29 



continuous supplies from below ; the root -system extends by 

 new suction-roots being formed ; fresh food-supplies are absorbed 

 and sent upwards through the soft sapwood; and active vegeta- 

 tion continues till autumn, the food-supplies transmitted to the 

 leaves being there subjected to the chemical action of light and 

 transformed under the processes of assimilation of atmospheric 

 carbon and preparation of carbo-hydrates, and the elaborated 

 food returned down through the cambium to form a new layer 

 of sapwood on the woody fibrous tissue and of bark on the 

 outer protective cuticle. By the end of the year the phenomena 

 of the previous season have been repeated ; ligneous matter has 

 been gradually produced during the period of active vegetation ; 

 and, as the zone deposited in summer is denser than that formed 

 in spring, this gives rise to the appearance of annual rings or 

 concentric zones of woody tissue ; new shoots and leaf-buds are 

 formed before the winter period of rest from active vegetation; 

 and the stem has grown in diameter. 



During the third year similar processes are repeated on a 

 more extensive scale. More roots and rootlets are formed ; 

 food- supplies are absorbed and elaborated in larger quantities ; 

 new cambial layers are formed on the wood and the bark, and 

 the horizontal and the perpendicular developments of tissue are 

 continued; and another annual ring is added to those of the 

 two previous years. And thus year after year the tree continues 

 to grow till it is felled, or till its active functions cease through 

 old age and death, or disease, or accident. 



The Nutrition and the Growth of Trees depend partly on 

 physical and climatic, and partly on chemical conditions. The 

 essential physical factors are Warmth and Light, and the 

 chemical factors Oxygen, Carbon-dioxide, Nitrogen, and Water, 

 together with mineral substances absorbed from the soil in the 

 form of soluble nutrient salts. Where all these factors are 

 combined favourably for the requirements of any individual 

 kind of plant, it can thrive well; but where any one factor 



