xlviii IXTRODUCTION TO BOTAXV. 



use of stipulas is, probably, to protect the leaves before their 

 developement. 



Tendrils 



Are abortive organs. Sometimes they ai-e floral stalks, at 

 other times, leaf-stalks, stipulas, or abortive branches. 



Spines, (thorns,) 



Are very often only abortive branches; example, the sloe- 

 tree : for if this tree be transplanted into a richer soil, its 

 spines are converted into branches. Prickles have been con- 

 sidered by some, as hardened hairs. 



1. Of Nutrition in Vegetables. 



In nutrition, vegetables assimilate apart of the solid, liquid, 

 or gaseous substances contained in the earth, or diffused 

 through the atmosphere. The absorption of these substances 

 is effected by suction. Water serves as a solvent for the 

 bodies which they are to assimilate. The force with which 

 the sap ascends from the root of a vine into the stem is greater 

 than the pressure of the atmosphere. 



2. Course of the Sap. 



The sap holds the nutritive principles in solution, and depo- 

 sits them in the different parts of the vegetable as it circulates 

 through them. The sap ascends through the woody layers, 

 and the lymphatic vessels of the wood, and the alburnum con- 

 vey that fluid : a coloured fluid will be absorbed, particularly 

 by those vessels, which are nearest to the medullary tube, but 

 not by the pith, or bark. The sap passing through the layers 

 of wood in its course upwards, communicates with the lateral 

 parts and branches of the stem, either directly, by the anasto- 

 mosis (peculiar union) of their vessels, or by diffusing itself 

 gradually through pores in the (parietes) sides of the canals, 

 which convey it. In the same sap vessel there are always 

 four different currents : an ascending and descending, and two 

 horizontal ones in different directions. 



