Leading Principles of the Growth of Trees. 1 7 



plants consume it and restore the oxygen. Connected with this, 

 there is another interesting proof of creative design. If there were 

 no carbonic acid in the air, plants could not grow ; but one-twenty- 

 five-hundredth part, as now exists, supplies food for vegetation, and 

 does not affect the health of animals and man. 



Leaves require sunlight to enable them thus to decompose car- 

 bonic acid. It does not go on in a dark room, or in the night. An 

 excess of oxygen in a plant makes it pale in color, and either sour 

 or insipid in taste ; an excess of carbon makes it dark green, high- 

 flavored, or bitter. Hence, a potato growing in a dark cellar is pale 

 or white ; hence the process of blanching celery and sea-kale to 

 remove the bitter taste. Hence also the reason that a potato much 

 exposed to the sun imbibes too much carbon, and becomes bitter. 

 Hence, too, strawberries and other fruits are more acid when hidden 

 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 which feed them are fully exposed to the 

 light, and receive a proper share of carbon. 



The sap, thickened, reduced in bulk, and prepared in the leaves, 

 then descends through the inner bark, forming a layer of fresh, half- 

 liquid substance, between bark and wood, called the cambium 

 most of which, by hardening, 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 deposited, may be 

 proved by an interesting experiment, performed by slitting the bark 

 of a young tree, lifting it up carefully, 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 inside ; and after 

 a lapse of years the concentric rings will be found to 

 correspond exactly with the time since the opera- 

 tion was performed. 



The descent of the forming wood in the inner bark 

 may be shown by tying a ligature around a growing 

 branch, or by removing a ring of bark. The down- 

 ward currents are obstructed, like that of a stream 

 by a dam, and the new wood accumulates above the 

 obstruction, and not below, as shown in Fig. 6. 



In Grafting, it is essential that some portions of 

 the cut surfaces uniting the stock and shoot should F i g . 6 . 



