THE planter's GUIDE. 



85 



multiplies or thickens the brandies and spraj, the cellular 

 tissue and cortical layers are thickened for the protection 

 of the proper vessels. The inner layers being always the 

 softest, the outer by this means gradually increase in 

 solidity and thickness, and become visibly indurated ; so 

 that with some trees it even sloughs, and splits into 

 chinks and fissures, as in the case of the Sycamore, the 

 Elm, and the Fir. In this condition, we may conceive how 

 well adapted such a mass of non-conducting matter is to 

 protect from cold the ascending, and still more the 

 descending or proper vessels, as already mentioned. 



In order to assist the reader in forming a clear concep- 

 tion of the great value of a proper thickness of bark to 

 trees intended for removal, it will be necessary to inquire 

 a little into the means by which the sap-vessels minister 

 to the sustenance of plants. In the warmer latitudes the 

 sap flows in certain plants during the whole year ; but in 

 those that are more temperate, the functions of vegetables 

 are suspended, or nearly so, during the winter season. 

 Early in the spring, however, it begins to rise in woody 

 plants, and continues to ascend till it reaches the extremi- 

 ties of the branches. This sap is absorbed from the soil 

 by the extremities of the capillary rootlets, and conveyed 

 upwards, through the vessels of the root, to the trunk. 

 In its ascent it rises only through the wood and the 

 alburnum, in tubes of various sizes, and is prepared or 

 elaborated by the leaves. That process, according to 

 some, is effected by means of an alternate contraction and 

 dilatation of the sap-vessels, and still more by a respira- 

 tion, perceptible and imperceptible, in the leaves, which is 

 peculiar to plants, whether woody or herbaceous, and by 

 the action of the atmosphere : but according to others it 

 is rather the exhalation from the leaves, than what is 

 properly their respiratory functions, that effects the 



