86 



THE PLANTEE's GUIDE. 



ascent of the sap. When this has taken place, the sap 

 is convert ed into the proper juice, or what has been by 

 some called cambium — that is, juice fitted for nutrition; 

 and it descends bj the returning vessels of the leaf-stalk, 

 and the longitudinal vessels of the rind, or inner bark. 

 Thus the circulation is carried on by a double process, 

 the ascending and the descending ; whereby the vessels 

 terminate downwards in absorbents, by which the fluids 

 are received, and they terminate upwards in exhalents, 

 by which those fluids are discharged. This doctrine of 

 the two ciuTents of sap was originally struck out by 

 Malpighi and Grew ; but the first who showed the organs 

 of communication between the two currents to be the 

 leaves was unquestionably Darwin — a discovery which 

 the ingenuity of Knight subsequently extended and con- 

 fii'med, and traced the existence of the circulation of the 

 sap. 



During the descent of the proper juice, it further 

 appears that each branch is nourished by the juice pre- 

 pared by itself, and that the surplus beyond what is 

 required for that pm^pose descends from the junction of 

 the branch with the stem, and contributes to the increase 

 of tlie stem, and at last of the roots which originally 

 supplied it. The descending juice is the efiicient and 

 proximate means employed by nature for the support and 

 nourishment of every part ; therefore, to say that a tree 

 is vigorous and healthy, is to say, in effect, that it has an 

 abundant supply of sap."' 



The true constitution and anatomy of plants was first 

 systematically brought forward, on the Continent, by 



* See Grew and Malpighi, Anat. Plant, passim. Darwin's Phytologia. De 

 Saussure, Encyclop. Method. — Wildenow, Prin. of Bot. p. 85. Knight, Philo- 

 Boph. Trans. 1803, 1806. Ellis, in art. Veget. Physiol, in Sup. to the 

 Encyclop. Britan. 



