Thomson's Lectures on Botany. 133 



own proofs of the matter are Chaptal's discovery of ligneous fibre in the 

 juice ; opinions drawn from experiments of Du Hamel, Hope, and Knight ; 

 from the circumstance of the upper lip of the wound on a tree protruding 

 faster downwards than the lower lip does upwards ; but, above all, by 

 " the simple fact, that isolated spots of bark and alburnum are formed on 

 decorticated stems." 



Now, without noticing in this place the m.otion, let us only consider the 

 nature of the sap. The lecturer has already described it as chiefly water, 

 drawn from the earth or air, and impregnated with various qualities exist- 

 ing in the plant j it is elaborated in the vessels and leaves, and thus changed 

 is returned either into the roots of herbs, or into the bodies of shrubs and 

 trees. If extracted from the plant, and submitted to chemical processes, 

 however changed by such means into its hardest concrete state, it still, in 

 all shapes (except only the residuum found by Chaptal), remains a homoge- 

 neous mass, entirely destitute of organisation. See it filling up a flaw or 

 knot in pine timber, existing many years before, and for ages after the tree 

 is felled, without change : placed once in the alburnum, the very place 

 where it should have been transmuted, but still maintaining its simple cha- 

 racter ; bearing the action of the saw and plane, and even the chissel, without 

 fracture, or decomposition, except by heat. I therefore doubt that ligneous 

 substance can be formed of the matured sap, but think that it originates 

 from the membrane, as already stated in the third lecture. Besides, if I 

 examine a seedling tree, I find both its root and stem formed of fibres ; or 

 a tree, when felled, the whole a fibrous structure ; a plank cut out of the 

 centre, shows whether it has begun to decay at the pith, and how much is 

 perfect timber ; two or three of the outer layers are (if oak) whiter than 

 the heart ; these are called by carpenters the sap of the timber, and the outer 

 one of them is called by botanists the alburnum ; on the outside of the albur- 

 num is the reticulated tissue of liber, and outside of all is the scabrous 

 bark. Now, all these different parts, except the exterior face of the bark, 

 are found to be composed oi fibres more or less consolidated: and if we 

 look at the crescive processes going on upon the exterior surface of the 

 plant, we find also, that their attachments and connections are all by means 

 of fibres or cellular matter, which, increasing in size and numbers, enlarge 

 the sustaining trunk. I conclude, therefore, that the sap invigorates, dis- 

 tends, and agglutinates the organs, but doubt whether any of them is 

 formed of it. 



On this part of the subject the lectui'er is perfectly candid in showing 

 that it is very differently understood by other physiologists. Darwin, and 

 Petit-Thouars after him, maintain that the alburnum is annually formed by 

 the descending radicles from every bud and shoot growing on the superior 

 parts of the tree. This hypothesis, though deserving of attention, is how- 

 ever scouted as being " wholly founded upon assumption." Be this as it 

 may, there are several appearances in vegetation, which cannot be ratio- 

 nally accounted for, unless something of the kind is admitted : and perhaps 

 this part of the subject should rather remain sub silentio, under the candid 

 admission of its obscurity (p. 121.), than be asserted in any thing like posi- 

 tive terms. 



Lecture 8. contains a most satisfactory description of " the origin and 

 attachment of branches and buds j" and here the lecturer, from the patient 

 and attentive investigation he has bestowed, is enabled to stand forth as 

 the disciple of experience, and advocate of truth. He proves by his figured 

 dissections, that buds, and of course branches, originate in the medulla, and 

 not on the alburnum, as asserted by other physiologists : and shows that 

 the medullary divergent rays so visible to the eye in timber, are the tracks 

 of the gems from the point where they are first formed, to the exterior 

 surface, where they either remain latent, or are developed accordingly as 



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