On the Anatomy of Vegetables. 1 75 



left. This filament is opake, brilliant, argenteous, and thick. 

 Its tranfverfal fection prefented to me fometimes a flat plate 

 or an elliptis, and fometimes even two filaments united by 

 an intermediate membrane; but I never could obferve the 

 aperture of a tube, as feveral authors have afTerted. The 

 furface is fometimes fmooth, fometimes unequal, and fome- 

 times porous. The fpirals of the tracheae are often fo clofe, 

 that when their arrangement is not difturbed, on breaking or 

 cutting, without precaution, the parts which conceal them, 

 they appear to be continued tubes {lightly ftriated. Malphigi 

 and Reichel fay that choked parts have been remarked in 

 the length of the tracheae ; and at firft I believed that I ob- 

 ferved the fame, but I have fince found that this was merely 

 an optical illufion. Thefe tubes exift in great numbers in 

 the herbaceous monocotyledons and dicotyledons, but efpe- 

 cially in the aquatic kinds, the tiflue of which is weaker : 

 they occupy the centre of the ligneous filaments in the mo- 

 nocotyledons ; in trees with two cotyledons they are feen 

 around the pith : in thefe they are often mixed, and con- 

 founded with the falfe tracheae. I never faw them in the 

 hard parts of vegetables, unlefs thefe parts had long been in 

 a ftate of foftnefs, which permitted ihe tracheae to expand : 

 this is what takes place in branches and twigs from which 

 the pith has difappeared. Thefe tubes have formed them- 

 felves when the medullary fubftance exifled. The trachea? 

 are not found in the length of the bark; they penetrate into 

 the petioles and leaves in the fame manner as the falfe tra- 

 cheae ; they every where acl: the fame part as the latter, and 

 do not contain thick juices but in plants where they are very 

 abundant, fome as of the lily kind. It is well known that, to 

 fee thefe organs with the naked eye, it is neceflary to take a 

 young, green, and foft branchy to twift and break it without 

 violence, that the tracheae may be unrolled without rupture : 

 if the two parts of the branch which have been divided be 

 then oppofed to the light, one can diftinguifh the half-un- 

 rolled filaments which proceed from the one part to the other, 

 and the fpirals are clofe or at a di (lance, according as the parts 

 are brought near to, or removed from, each other. They 

 unroll themfelves or contract in the fame manner in leaves 

 which have been torn. The leaves, however, of the butomus 

 umbellatus exhibit a contrary phaenomenon ; the tracheae, 

 which in this plant are exceedingly numerous, when once 

 unrolled no longer contract themfelves*. 



Let us now return to the large tubes in general. The tli- 

 vifion into ample tubes, porous tubes, falfe tracheae, and 



tracherp, 



