On the Anatomy of Vegetables. 1 73 



and they exift only becaufe there is a lacuna in the mem- 

 branes. Such is the extreme fimplicity of the organization 

 of vegetables, that all the difference obferved in them i> 

 merely confined to fome modifications in the cellular tidue.. 

 But the fides of thefe large tubes, being continually moiitened 

 by the fluids imbibed by the plant, gradually afiume more 

 confidence, and ieparate from the reft of the tifiue when their 

 folidity furpaflcs that of the furrounding membranes. I could 

 never obferve large tubes in muftirooms, lichens, and fuci, 

 even when I employed a microfcope; but it is fufheient to 

 have good fight to be able to diftinguifh the aperture of thefe 

 canals" on the tranfverfal feclion of the Items, the branches, 

 and roots, of feveral monocotyledons and dicotyledons. In 

 the former, they are always found in the centre of the lig- 

 neous filaments, and fometimes they compofe the major part; 

 in the fecond they are distributed often, as it were, at ran- 

 dom in the wood ; fometimes alfo they form groups placed 

 very regularly at certain distances, or they are ranged in con- 

 centric zones. They are exceedingly numerous, in particu- 

 lar around the medullary canal. They are found alfo in the 

 bark. If their progrefs be followed they will be feen to have 

 their origin in the root, to enter the trunk, and to rife pa- 

 rallel to each other: then to unite, to feparate and deviate 

 from their vertical direction, to penetrate the bud which is 

 formed at the furface of the bark; to lengthen with it, and 

 to ditiribule themfelves throughout all its ramifications, to 

 pafs from the branch into the ligneous filaments the bundle 

 of which compofes the petiole, and to divide themfelves 

 among the lartre fibres of the leaves as the arteries and veins 

 diftnbute themfelves in the human body. They may be ob- 

 ferved alfo in the fibres of the perianthes, the filaments of 

 fome ftamina, the piftils, and the ligneous filaments which 

 traverfe the pulp of fruits. Scarcely is the embryo formed 

 when thefe tubes are obferved. In this infancy of the vege- 

 table they are not concealed by the wood, which does not 

 yet exift: the fubftance deftined to produce it is then in a 

 ilate of fluidity, which permits the obferver to examine the 

 parts which it covers. It is not yet the proper place for 

 (peaking of the vegetable chyle produced by the fluids aflimi- 

 latetl in the vetiels of the plant, I (hall recur to that fubjecvt 

 fiereafter. The large tubes form fometimes medullary radii, 

 as I have obferved in the equifetum or horfe-tail ; but, in my 

 opinion, this cafe is rare. 



The large tubes are of four kinds : fimple tubes, porous 

 tubes, fa lie trachea;., and tracheae. Thefe are modifications 

 pf the fame organ. 



I ft, Simple 



