BARK. lOY 



Qiidergone in the parenchyma of the leaves the changes -we 

 have described, is carried from the leaves into the bark, de- 

 ecending through the tissues, and also flowing laterally along 

 the medullary rays into the stem ; or in other words, it is con- 

 veyed by a system of vessels betw^een the lihe7\ or inner layer 

 of the bark, and the alburnum^ or young wood ; here it con- 

 tributes both to the formation of an outward layer of new 

 .wood and an inward layer of new bark; extending also from 

 the extremity of the roots to the upper extremity of the plant, it 

 furnishes materials for the formation of new buds and radicles. 



If a ring be cut through the bark of a tree, the cambium, or descending juice, will 

 be arrested in its course, and accumulating around the upper edge of tlie bark, will 

 cause a ridge or an annular protuberance. Tliis vegetable blood being thus pre- 

 vented from having access to the lower part of the plant, the roots cease to grow, 

 the sap ascends but feebly, and in two or three years the tree dies. If the incision 

 be not made too deep, the wound will soon heal by the union of the disconnected 

 bark, and the circulation of the cambium proceed as before. This experiment 

 proves the importance of this fluid to the existence of the plant. 



124. Proper Juices. — This division comprehends all the fluids 

 furnished by the plant, except the sap and cambium, as oils, 

 gums, &c. These are the product of the cambium, as, in the 

 animal system, tears are secreted from blood. The secretions 

 carried on by the vegetable glands from the cambium are of 

 two kinds : 1st, such as are destined to remain in the plant, as 

 milk, resins, gums, essential and fixed oils ; 2d, such as are des- 

 tined to be conveyed out of the plant ; these consist chiefly of 

 vapors and gases exhaled from flowers, and may, perhaps, more 

 properly be called excretions than secretions. 



LECTUEE XX 



PHYSIOLOGICAL VIEWS. BARK, W^OOD, AND PITH. GROWTH OF A DI- 

 COTYLEDONOUS PLANT. GROWTH OF A MONOCOTYLEDONOUS PLANT. 



125. We have exhibited the minute discoveries made by 

 the help of the microscope in the solid parts of the vegetable 

 substances, and noticed those important fluids^ the circulation 

 of which appears to constitute the life, and produce the growth 

 of plants. We have now to consider the solid parts already 

 described, as composing the body of the vegetable, and collect- 

 ed under the three forms of Bark^ Wood^ and Pith. 



Bark. — ^The harJc^ in exogenous plants, consists of the epider- 

 mis, cellular integument., and cortex. 



Importance of this fluid— Effect of cutting a ring through tlie bark of a tree.— 124. Proper juices.- 

 125. Division ol tlio stolid paru of the plant— Bark. 



