Book I. PROCESS OF VEGETABLE DEVELOPEMENT. 243 



hour of its birth was no thicker than a pin, in a few months acquires the diameter of an inch, or more. 

 This arises from the successive superposition of the bundles of fibres which are created upon the develope- 

 ment of each leaf, and of everv leaf-bud. The latter makes its tirst appearance under the form of a green 

 point, which originates from the inner layers of the ligneous body, which it traverses, and penetrates into 

 the bark. A short time after its tirst appearance, it may be perceived that the bud is surrounded by a 

 portion of woody fibre, which passes downwards, covers over the wood previously formed, and thus forms 

 a new layer. The existence of this it is easy to demonstrate ; for the tibres of the haves separate easily 

 from the' wood, but the leaf-buds, when broken off, evidently arise from the interior of the wood. All the 

 new parts formed bv the leaf-bud soon become so completely identified with the old wood, that, after a 

 short period, no marks of separation remain. " (London Encyclopedia, art. Botany.) 



1578. Conversion of the alburnum into perfect wood. In consequence of the increase of the trunk by 

 means of the regular and gradual addition of an annual layer, the layers, whether of wood or of bark, are 

 necessarily of different degrees of solidity in proportion to their age, the inner layer of bark and the 

 outer layer of wood being the softest; and the other layers increasing in their degree of solidity till you 

 reach the centre on the one hand, and the circumference on the other, where they are respectively the 

 hardest, forming perfect wood or highlv indurated bark, which bark sloughs or splits into chinks, and falls 

 off in thick crusts, as in the plane tree," fir, and birch. What length of time, then, is requisite to convert 

 the alburnum into perfect wood, or the liber into indurated bark ; and by what means are they so con- 

 verted? There is no fixed and definite period of time that can be positively assigned as necessary to the 

 complete induration of the wood or bark, though it seems to require a period of a good many years before 

 any particular layer is converted from the state of alburnum to that of perfect wood ; and perhaps no layer 

 has received its final degree of induration till such time as the tree has arrived at its full growth. The indu- 

 ration of the alburnum, and its consequent durability, are attributed by many to the loss of sap which the 

 layer sustains after the period of its complete developement, when the supply from the root diminishes, 

 and the wa.-te by evaporation or otherwise is still kept up, inducing a contraction or condensation of its 

 elementary principles which augments the solidity of the layer, in the first degree, and begins the process 

 that future years finish. But Knight believes the induration of the alburnum, as distinguishable in the 

 winter, to b'e owing rather to some substance deposited in it in the course of the preceding summer, which 

 he regards as being the proper juice in a concrete or inspissated state, but which is tarried off again by the 

 sap as it ascends in the spring. 



1579. Circulation of vegetable juices. After the discovery of the circulation of the 

 blood of animals, phytologists, who were fond of tracing analogies between the animal 

 and vegetable kingdoms, began to think that there perhaps existed in plants also a circu- 

 lation of fluids. The sap was supposed to be elaborated in the root. The vessels in 

 which it was propelled to the summit of the plant were denominated arteries ; and the 

 vessels in which it was again returned to the root were denominated veins. Du Hamel, 

 while he admits the ascent of the sap, and descent of the proper juice, each in peculiar 

 and appropriate vessels, does not, however, admit the doctrine of a circulation, which 

 seems, about the middle of the last century, to have fallen into disrepute. For Hales, who 

 contended for an alternate ascent and descent of fluids in the day and night, and in the 

 same vessels, or for a sort of vibratory motion, as he also describes it, gave no countenance 

 whatever to the doctrine of a circulation of juices. But the doctrine, as it appears, has 

 been a<rain revived, and has met with the support of some of the most distinguished of 

 modern phytologists. Hedwig is said to have declared himself to be of opinion, that plants 

 have a circulation of fluids similar to that of animals. Corti is said to have discovered a 

 species of circulation in the stem of the Chara, but confined, it is believed, within 

 the limits of the internodia. Willdenow has also introduced the subject, and de- 

 fended the doctrine (Principles of Botany, p. 85.) ; but only by saying he believes a cir- 

 culation to exist, and that it is impossible for the leafless tree to resist the cold if there is 

 not a circulation of fluids. Knight has given his reasons somewhat in detail ; and 

 though his doctrine of a circulation should be false, yet the account which he gives of the 

 progress and agency of the rap and proper juice, short of circulation, may be true. The 

 sum of the account is as follows : — When the seed is deposited in the ground under 

 proper conditions, moisture is absorbed and modified by the cotyledons, and conducted 

 directlv to the radicle, which is by consequence first developed. But the fluid which has 

 been thus conducted to the radicle, mingling no doubt with the fluid which is now also 

 absorbed from the soil, ascends afterwards to the plumelet through the medium of the 

 tubes of the alburnum. The plumelet now expands and gives tire due preparation to the 

 ascending sap, returning it in its elaborated state to the tubes of the bark, through 

 which it again descends to the extremity of the root, forming in its progress new bark and 

 new alburnum ; but mixing also, as he thinks, with the alburnum of the former year, 

 where such alburnum exists, and so completing the circulation. 



1580. Decomposite organs. To the above brief sketch of the agency of the vital 

 principle in the generation or growth of the elementary and composite organs, there now 

 remains to be added that of the progress and mode of the growth of the decomposite or- 

 gans, or organs immediately constituting the plant, as finishing the process of the vege- 

 table developement. This will include the phenomena of the ultimate developement of 

 the root, stem, branch, bud, leaf, flower, and fruit. 



15S1. The root. From the foregoing observations and experiments, it appears that the roots of plants 

 or at least of woody plants, are augmented in their width bv the addition of an annual layer, and in their 

 length bv the addition of an annual shoot, bursting from the terminating fibre. But how is the develope- 

 ment of 'the shoot effected ? Is it by the introsusception of additional particles throughout the whole of 

 its extent; or only by additions deposited at the extremity? In order to ascertain the fact, with regard 

 to the elongation of the root, Du Hamel instituted the following experiment : — Having passed several 

 threads of silver transversely through the root of a plant, and noted the distances, he then immersed the 

 root in water. The upper'threads retained always their relative and original situation, and the lowest 

 thread, which was placed within a few hues of the' end, was the only one that was carried down. Hence 

 he concluded that the root is elongated merely bv the extremity. Knight, who from a similar experiment 



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