CH\P. III. ELEMENTARY ORGANS. 345 



his investigations even into the interior of the 

 tubes themselves, in which he appears to have made 

 some minute discoveries. He describes the fibres of 

 the vegetable, whether vertical and horizontal, as 

 constituting tubes whose diameter he attempted to 

 measure, and whose interior he found to be lined 

 with a sort of fine down.* It is probable, how- 

 ever, that he was indebted for part of his discoveries 

 to microscopical deception. But it must be con- 

 fessed that the accuracy of his observations is in 

 some degree corroborated by those of Du Hamel, Du Ha- 

 who having contrived, by means of coloured injec- 

 tion, to render conspicuous the vascular structure of 

 the fibres of several different species of reed, de- 

 scribes them as being enveloped in a medullary 

 substance, and lined on the interior surface with a 

 fine down.-j- Tournefort describes the same ap- 

 pearances as visible in the longitudinal tubes of the fort ' 

 stem of the Potamogeton and Nymphcea; in which 

 last they are easily detected, owing to the large 

 diameter of the tubes, though I should be inclined 

 to denominate them internal spines rather than a 

 down, they appear so large and strong. 



But the tubular structure of the fibres is not so As 

 readily distinguished in the stem of woody plants, ^" 

 though the evidence of its existence is sufficiently P lanls - 

 satisfactory. It cannot be denied that wood is, at 

 least, permeable to water, as is plain from the fol- 

 lowing facts. A wedge of wood driven into the 

 * Arcana Nutura?, p. 12, f Phys. cjo< Arb. liv. i. chap. i. 



