174 ?l the Anatomy of Vegetable*. 



I ft, Simple tubes. — The fides of thefe tubes are perfectly 

 whole, neither pores nor fifTures are obferved in them : they 

 generally contain refinous or oily juices, known under the 

 denomination of proper juices. Thefe tubes are very re- 

 markable in green trees, in euphorbia, periploca, and, in ge- 

 neral, in all plants the juices of which are thick. They are 

 more numerous and more vifible in the bark than in any 

 other part. 



sd, Porous tubes. — The fides of thefe tubes are perforated 

 with fmall pores fimilar to thofe mentioned in the article on 

 the cellular tiflue, with this difference, that they are much 

 more numerous, and that, inftead of being difieminated by 

 chance, and without order, as is often the cafe, in the cells, 

 they are diftributed, in regular and parallel feries around the 

 tubes. Thefe tubes do not appear to be fo peculiarly deftined 

 as the preceding to contain refinous or oily juices. They are 

 very numerous in hard wood, fuch as the oak. 



3d, The falfe trachea. — Thefe tubes are interfe&ed in a 

 tranfverfal direction with parallel flits, which from their ap- 

 pearance might induce us to believe that they are formed of 

 rings placed one above the other, or of filaments twifted in a 

 fpiral form ; but they cannot be unrolled, or feparated into 

 diftincl. rings ; and befides, with a little attention, one may 

 difcover the continuity of the membrane, and confequently 

 the place where the flits end. Thefe, then, are porous tubes, 

 but their pores are much larger than thofe of the preceding. 

 I muft even obferve, that the edge of the flits is furnifhed 

 with a roll fimilar to that which furrounds the fmall pores. 

 Thefe tubes are deftined for the fame purpofes as the porous 

 tubes ; but in general they are found in thofe kinds of wood 

 which are lefs hard and com pact, and even in herbaceousplants: 

 I have obferved them in a great number of the monocotyle- 

 dons. The centre of the lycopodia prefents a thick cylinder 

 compofed, in a great meafure, of veffels of this nature. Ferns 

 contain alfo a great many in their ligneous filaments. Dico- 

 tyledons are likewife provided with them. They are exceed- 

 ingly numerous in the vine, the wood of which is foft and 

 porous. 



4th, The trachea. — Want of experience has affigned to 

 thefe tubes, which have not been iufficiently obferved, a de- 

 nomination confecrated by cuftom. The tracheae of plants 

 have a relemblance in their form to the tracheae of infe6ts, 

 and it has thence been concluded, on too flight grounds, that 

 thefe tracheae in the former as well as in the latter muft be 

 the organs of refpiration. The vegetable trachea is a tube 

 formed of a filament twifted into a fpiral form from right to 



left. 



