io Principles OP TitE Growth of trees. 



Hence, strawberries and other fruits are more acid when hid- 

 den by leaves or in cloudy weather ; and apples on the thickly- 

 shaded part of an unpruned tree are more sour and imperfect 

 than where, by good pruning, the leaves v/hich feed them are 

 fully exposed to the light, and receive a proper share of car- 

 bon. 



The sap, thickened, reduced in bulk, and containing organ- 

 ized matter, then descends the branches and trunk, forming a 

 layer of fresh substance, between bark and wood, called the 

 cambium — most of which constitutes a new layer of wood — a 

 small part making a new layer of bark. The annual deposits 

 of new wood form distinct concentric rings, by which the age 

 of the tree may be counted when the trunk is cut through. 

 That this is the mode by which wood in exogenous trees is de- 

 posited, may be proved by an interesting experiment, per- 

 formed by slitting the bark of a young tree, lifting it up care- 

 fully, and then slipping in between wood and bark a sheet of 

 tin-foil, and binding the bark on again. The bark will deposit 

 layers of wood outside the tin-foil, and none in- 

 side ; and after a lapse of years the concentric 

 rings will be found to correspond exactly with 

 the time since the operation was performed. 



The descent of the elaborated sap in the 

 inner bark may be shown by tying a ligature 

 around a growing branch, or by removing a 

 ring of bark. The downward currents are ob- 

 structed, like that of a stream by a dam, and 

 new material accumulates above the obstruc- 

 tion, and not below, as shown in Fig. 7. 

 FIG. 7.— Effects o£ I^ Grafting, it is essential that some portions 

 a Constriction of the cut Surfaces uniting the stock and shoot 

 upon the Growth ghould be placed so accurately together that 



of a Sapling. ^ jo 



the sap may flow up through the alburnum or 

 sap-wood from the stock to the shoot, and that the cambium 

 of the shoot may grow to that of the stock. When this union 

 takes place, the rest of the cut faces, even if some distance 

 apart, are soon cemented by the newly-forming wood, which 

 fills all the vacant space. 



In Budding, the newly-set bud is cemented to the wood of 

 the stock by the cambium, which hardens and fastens it. The 



