THE LIFE HISTORY OF A TREE n 



a large volume of water must therefore pass through the tree 

 in order that it may obtain a sufficient supply of mineral 

 matters. The excess water is evaporated into the air through 

 the stomata, leaving the mineral matters behind. This 

 evaporation from the leaf surface is called transpiration. On 

 a hot day transpiration is rapid, and if at the same time the 

 soil is dry and the roots do not get enough water, the leaves 

 fade. A great deal of power must be required to force 

 the water from the roots up to the top of a tall tree : what 

 this power is cannot be explained exactly, but it is known 

 that there is a considerable pumping action by the roots 

 and also a sucking action due to transpiration from the 

 leaves, and these two forces, especially the latter, may be 

 perhaps the main factors in producing the flow of water up 

 the stem. 



The growth of a tree. The tree forms sugar in the way 

 described, and from it, together with the mineral matters, 

 makes new wood and other material. The wood consists of 

 cells which formerly contained protoplasm, the living sub- 

 stance of which plants and animals are essentially composed. 

 Concentric layers of these cells are laid on in the form of 

 a thin coat over the whole tree between the older wood and 

 the bark. The new twigs grow in length by a kind of 

 stretching out from the buds ; this stretching only goes on in 

 the first year of the life of the twigs and then ceases, buds 

 being formed from which the next year's twigs shoot out. 

 The older twigs, the branches, and the stem grow in thickness 

 only, and do not increase in length ; they grow thicker be- 

 cause new wood is laid on over them each year. Between the 

 wood and the bark there is a very thin layer of living cells 

 called the cambium ; this, obtaining nourishment from the 

 sugary sap, increases and grows in thickness, its inner layers 

 giving rise to wood, and its outer layers to bark, while be- 

 tween these two there yet always remains a thin layer of 

 cambium. The multiplication of cells inwards which turn 



