66 OP THE SAP, 



and useful to the unexhausted constitutions of chiMrenr 

 The same principle accounts for the occasional Jlowing 

 of the sap in autumn after a slight frost. Such a prema- 

 ture cold increases the sensibility of the plant to any 

 warmth that ma}'- follow, and produces, in a degree, the 

 same state of its constitution as exists after the longer 

 and severer cold of winter. Let me be allowed a fur- 

 ther illustration from the animal kingdom. Every body 

 conversant with labouring cattle must have observed 

 hovv much sooner they are exhausted by the warm days 

 of autumn, when the nights are cold, than in much hot- 

 ter weailier in summer, and this is surtly from the same 

 cause as the autumnal flowing of the vegetable sap.(5) 



The sap, or lymph, of most plants when collected in 

 the spring as above mentioned, appears to the sight and 

 taste little else than water, but it soon undergoes fermen- 

 tation and putref;iction. Even that of the Vine is 

 scarcely acid, though it can hardly be obtained without 



(5) [In addition to the above explanation of the flowing' 

 of the sap, we may subjoin one which has been suggested, but 

 not enlarged 0:1, by Mr. Knight. In the spring of the year the 

 sap begins to ascend from the root sometime before the expan- 

 sion of the buds. As at this time there are no leaves, flowers, 

 8cc. on which the sap may be expended, the trunk, becomes 

 overcharged with it, and will readily bleed if wounded. After 

 the leaves are developed, and the growth of the new layer of 

 wood has commenced, all the sap from tlie trunk is required to 

 atfurd the material for the new growth, and to supply the pro- 

 digious expenditure by perspiration from the leaves. At this 

 period no sap flows from incisions in the trunk. In autumn after 

 a frost has taken place, the functions of the leaves are suddenly 

 checked, the s^ip is ug dn if.stncted to the tru;iK., the vessds are 

 again overcharged with fluid, and will bleed again if divided.^ 



