Exogenous Tree. 479 



(i. 340), that it is not easy to conceive how additional ringlets 

 of wood should be formed by a merely horizontal dilatation of 

 the vessels; but rather by the shooting of the longitudinal fibres 

 lengthways under the bark, as young fibrous shoots of roots do 

 in the solid earth.' Whatever claim these authors may have 

 to suggesting this idea, it is certain that it is chiefly known at 

 the present day, in consequence of the writings of M. du Petit 

 Thouars, and one or two others who have followed him. This 

 doctrine leads to a curious view of the nature of plants in gene- 

 ral ; a subject upon which this is no place to enter fully, but 

 of which a concise explanation is necessary, for the attainment 

 of the end I have in view in this communication. 



A plant is to be understood as a mass of individuals, each 

 having its own peculiar system of life, growing together in a 

 definite manner, and having a common organization, but 

 nevertheless capable of vegetating independently, and not un- 

 frequently separating spontaneously from each other. These 

 individuals are buds, each of which is perfect in itself, and 

 exactly the same as all the others of the same plant. They 

 are combined by means of a fibro-cellular substance called 

 bark, which is to be understood as being composed of the cel- 

 lular integuments of as many individuals as the plant may have 

 developed buds. As the act of vegetation consists in the 

 development of a germinating body in two opposite directions, 

 the one upwards, as stem, the other downwards, as root, every 

 bud, when it begins to grow, must be subject to this law, pro- 

 vided it is the independent being which it has been represented 

 to be. And, in fact, if a bud is separated from the system to 

 which it belongs, it does follow this law of development, as is 

 well known to gardeners, from their practice of propagating 

 plants by buds and eyes. Now, if buds, when in a state of 

 combination, undergo the same kind of development as when 

 isolated, as it is reasonable to suppose, it will be found that the 

 fibrous and vascular tissue of the wood and bark, which always 

 descends from the buds, is really their roots ; and that, conse- 

 quently, the concentric circles of the wood and bark of dicoty- 

 ledonous trees are congeries of roots formed by the annual 

 development of buds upon the surface of the plant. It is well 

 known that, whatever the origin of the wood and bark may be, 



