S2 



HISTORY OF THE VEGETABLE KINGDO.M. 



tlons; 1st, In thickness, by tlie formation of 

 new layers between the bark and alburnum ; and 

 2dly, In hreadth, by the lateral development of 

 the new layer and the formation of new bundles 

 of fibres. This growth, in the direction of the 

 thickness and breadth, takes place ec[ually in 

 the roots and stems. 



It was on the stem of Clematis Vitalha that 

 Dutrochet made his first observations. When 

 the extremity of a young branch of that plant 

 is cut across, it is found to be composed of six 

 bundles of longitudinal fibres, separated from 

 each other by meduUary rays or spaces of con- 

 siderable breadth. By degrees, and in the pro- 

 gress of vegetation, there forms at the centre of 

 each medullary space a new bundle of longitu- 

 dinal fibres, which presently acquires the same 

 size as the six original bundles, so that, by the 

 end of the first year, the stem is found to be 

 composed of twelve bundles of fibres, separated 

 by an equal number of meduUary rays. 



In the course of the second year, each of the 

 six original bundles is divided into three by the 

 median production of a new bundle of longitu- 

 dinal fibres, separated from the other two, 

 between which it has been developed, by two 

 imperfect meduUary rays, which do not reach 

 the central medulla. On the other hand, the 

 six other secondary bundles of the first year 

 divide each into two, by the formation in their 

 middle of a new imperfect medullary ray. From 

 this there results, that, at the end of the second 

 year, there are thirty bundles of fibres, separated 

 from each other by an equal number of medul- 

 lary rays or spaces, of which twelve only, viz., 

 those which existed at the end of the first year, 

 are complete, and establish a direct communica- 

 tion between the external and the internal 

 medulla. 



If we attend to the manner in which the 

 bundles of longitudinal fibres have been multi- 

 plied, we shaU see that the growth has taken 

 place in a lateral direction ; for the median pro- 

 duction of new bundles of fibres at the centre of 

 the meduUary rays, or that of new meduUary 

 rays at the centre of the bundles of fibres, would 

 necessarily produce a lateral dilatation, and con- 

 sequently increase the width of the circular 

 layer in which this development had been 

 effected. This lateral dilatation was first per- 

 ceived by the able experimenter, whose observa- 

 tions we here relate. 



The growth in breadth stops in the parts the 

 moment they become solid. Thus it no longer 

 takes place in the woody layers ; but it continues 

 in the bark, and it is thus that it aUows the 

 woody layers to increase in tMckness. 



The growth in breadth takes place in the roots 

 also, as we have already said ; but, in that organ, 

 it always commences by the median production 

 of new meduUary rays at the centre of the 



bundles of fibres. Subsequently, these new 

 meduUary spaces themselves give rise to other 

 collections of fibres. 



From what has been said above, it wiU be 

 seen that the organic elements of vegetables have 

 a natural tendency to the median production. 

 Thus the bundles of fibres tend to produce new 

 meduUary rays in their middle part, and, on the 

 other hand, the medullary rays tend to produce 

 new bundles of longitudinal fibres. 



Having stated Dutrochet's opinion respecting 

 the growth in breadth, we now proceed to give 

 an account of his ideas on the development in 

 thickness. The woody layers of new formation 

 which are developed each year, are separated 

 from the old ones by a thin layer of central 

 medulla. These layers of medulla, which se- 

 parate the woody layers from each other, are 

 not always easily perceived ; but they are very dis- 

 tinct in some trees, for example, in Rhus typhina, 

 where their darker colour distinguishes them at 

 first sight from the layers of wood, which are 

 lighter. In spring, the growth in thickness 

 always commences by the formation of this thin 

 layer of cellular or medullary tissue. Soon 

 after, in consequence of the faculty which it 

 possesses of giving rise to longitudinal fibres, 

 this layer of pith produces vessels which smTound 

 it, and thus forms a kind of medullary canal, 

 which is destined at a later period to become the 

 new woody layer. 



In this theory we see the important function 

 which the author attributes to the pith. It be- 

 comes the essential agent of the growth in 

 diameter, ss it gives rise to the vessels which are 

 subsequently to form the new layer of wood. 



The same phenomena take place in the liber. 

 Each of its folds is separated by a thin layer of 

 ceUular tissue, which belongs to the cortical 

 meduUa, and by means of which the annual 

 growth is effected. 



Theory of engrafting. The most natural and 

 most easy means of multipUcation in vegetables 

 is undoubtedly by seeds, and it is that by which 

 the vegetables dispersed over the surface of the 

 globe are naturaUy renewed ; but there are others 

 wliich art frequently employs to perpetuate and 

 multiply certain races or varieties of trees whicli 

 cannot be propagated by seed. The processes 

 here alluded to, are the propagation by layers, 

 by slips, and by grafts. We shaU state the 

 theory of these three operations in a general 

 manner, and with reference to vegetable physi- 

 ology. An__aocount of the practical art of en- 

 grafting wiU be given afterwards. 



\st. Propagating by layers is an operation by 

 which the base of a young branch is surrounded 

 with earth, and made to shoot forth roots, before 

 it is separated from the parent stock. Some- 

 times this operation is performed upon the lower 

 branches of a young shi-ub which are bent 



