22 



mSTORY pP THE VEGETABLE KINGDOM. 



named tlie herbaceous envelope. Its colour is 

 generally green in young stems. It covers tlie 

 ti-unk, the brandies and their divisions, and fiUs 

 up the spaces which exist between the ramifica- 

 tions of the nerves of the leaves. To this, Du- 

 trochet applies the name of the outer medulla, 

 in opposition to that of inner medulla, which he 

 gives to the pith. Its colour is not derived from 

 the cellular tissue of which it is composed, but 

 is owing to the small grains of globuline, situated 

 in the walls of the cellules, and which Dutro- 

 chet considers as nervous corpuscules. 



The herbaceous envelope, or outer medulla, 

 frequently contains the proper juices of vege- 

 tables, which are enclosed in particular canals or 

 reservoirs. It is readily repaired on the stem 

 of woody vegetables ; but this phenomenon does 

 not take place in annual plants. It appears to 

 have an organization and uses similar to those 

 of the pith contained in the medullary tube. 

 When this herbaceous envelope acquires great 

 thickness, and peculiar physical qualities, it 

 constitutes the part known by the name of cork 

 in the cork tree, {quercus suber) and some other 

 plants. The herbaceous envelope is the seat of 

 one of the most remarkable chemical phenomena 

 which vegetable life presents : in its interior, 

 and that by a cause which it is difficult to un- 

 derstand, the decomposition of the carbonic acid 

 absorbed from the air by the plant, is efl^ected, 

 the carbon remaining in the interior of the vege- 

 table, while the oxygen that has been disengaged 

 is thrown out. It is to be remarked, however, 

 that this decomposition takes place only when 

 the plant is exposed to the rays of the sun, 

 whereas the carbonic acid is thrown out unde- 

 composed when the vegetable is withdrawn 

 fi'om the influence of that luminary. This organ 

 is partly renewed each year. It also performs a 

 very important part in the process of vegetation. 

 At the return of summer, it incites the sap to 

 ascend towards the buds, and thus becomes one 

 of the most powerful agents in producing their 

 growth and development into leaves. 



The herbaceous envelope is very easily dis- 

 covered on the young branches of a tree, it being 

 the part exposed when the epidermis is removed. 



The cortical layers, or outer bark, do not al- 

 ways exist, and are occasionally so slightly de- 

 veloped, and so little distinct from the liber, that 

 it becomes very difficult to recognise them. 

 They are placed beneath the herbaceous envelope, 

 and are applied upon the outermost layers of the 

 liber, from which they can hardly be distin- 

 guished. In no vegetable are they more appar- 

 ent, or more remarkable for the singular disposi- 

 tion of the tissue of which they are composed, 

 than in the lace-tree, in which they form several 

 layers above each other, which, on being stretched 

 out, hear a perfect resemblance to some kinds of 

 linen, or represent lacework of pretty regular 



texture. In most plants, however, it is difficult 

 to distinguish this part from the liber. 



The liber, or inner baric, or true bark, as it is 

 sometimes called, lies immediately in contact 

 with the alburnum, or first circle of woody fibre. 

 It is composed of a vascular network, the elon- 

 gated meshes of which are filled with cellular 

 tissue. It is seldom that it can be easily separ- 

 ated into distinct laminiE, or plates, which have 

 been compared to the leaves of a book,* but this 

 effect may almost always be obtained by mace- 

 ration. 



The different laminae of which the liber is 

 composed, and which have been successively 

 formed, have thin layers of cellular tissue inter- 

 posed between them. When the liber is macer- 

 ated, this cellular tissue is destroyed, and allows 

 the laminse to be separated. 



Like all other parts of the bark, the liber is 

 capable of being replaced when it has been 

 removed. Before it can be reproduced, however, 

 the part from which it has been detached must 

 be guarded from the contact of air. This im- 

 portant fact we owe to Du Hamel. That excellent 

 naturalist, to whom vegetable physiology is in- 

 debted for so many happy discoveries, removed 

 a portion of bark from a vigorous tree in full 

 vegetation. He secured the wound against the 

 contact of air, and presently saw exuding from 

 the surface of the woody body, and the edges of 

 the bark, a viscid substance, which, spreading 

 over the wound, acquired consistence, became 

 green and cellular, and reproduced the portion 

 of liber that had been removed. 



To this viscid substance, which exudes from 

 the denuded parts to reproduce the liber, Grew, 

 and after him Du Hamel, gave the name of cam- 

 bium. Several authors are of opinion that the 

 cambium is nothing else than the descending 

 and elaborated sap. This opinion becomes the 

 more probable, when we reflect that this viscid 

 fluid performs exactly the same functions in the 

 animal economy as those generally attributed to 

 the descending sap, which is conveyed bv the 

 same parts. 



Whatever be the origin of the cambium, it 

 performs a very important part in the growth 

 of the stem. For, in all the theories that have 

 been advanced with the view of explaining 

 that phenomenon, its presence is indispensable, 

 as we shall presently show, when we come to 

 treat of the growth of dicotyledonous stems. 



Numerous experiments prove that the liber is 

 absolutely necessary for vegetation. A graft 

 does not succeed unless its liber be in contact 

 with that of the tree on which it is inserted; 

 and a slip, whose lower part is destitute of liber. 



* Before the manufacture of paper, the inner bark of 

 some trees was used as a substitute; hence, tlie deriva- 

 tion of lihar, a booli. 



