288 The Rev. P. Keith on the Internal Structure of Plants. 



merely en passant, as the functions of the several organs of the 

 plant do not come within the scope of our present inquiry. 



The third species of tubes conducting sap is the false spiral 

 tubes. According to Mirbel they derive their spiral appear- 

 ance merely from being cut transversely by parallel fissures. 

 They cannot, like the true spirals, be uncoiled, and are hence 

 fitly denominated false spirals. They are said to abound in 

 the Lycopodia and in Ferns, and in the soft parts of the Vine. 

 But what says Dutrochet? He says that what Mirbel took to 

 be parallel fissures are merely globules arranged as in the 

 porous tubes; that the spires may be uncoiled by means of 

 the long-continued action of nitric acid destroying the bond 

 of agglutination; and that the vessels in question are, after all, 

 veritable spirals*. 



The fourth species of tubes conducting sap is the mixed 

 tubes, that is, tubes combining in a single individual two or 

 more of the foregoing species. Mirbel exemplifies them in 

 the case of the Flowering Rush ; but Dutrochet says there are 

 no such vessels as mixed tubes in any plant whatever, unless 

 you choose to regard as such tubes having some globules ar- 

 ranged in transverse rows, and some scattered. Yet these 

 accidental varieties are not enough to constitute species. 



The fifth species of tubes conducting sap is the cellular 

 tubes, that is, tubes composed of a succession of elongated 

 cells united like those of the cellular tissue. Individually 

 they may be compared to the stem of the grasses, which is 

 formed of several internodia separated by transverse dia- 

 phragms ; and collectively, to a united assemblage of parallel 

 and collateral reeds. Dutrochet admits the existence of such 

 vessels, but he designates them by a different name, and finds 

 them assuming not merely a longitudinal but also a transverse 

 direction, making part of the medullary rays. Like the cells 

 out of which they are formed, they are furnished with glo- 

 bules. 



The only species of proper vessels that M. Mirbel describes 

 is the simple tubes. They are the largest of all vegetable 

 tubes, and are formed of a thin but entire membrane, without 

 any perceptible fissure or disruption of continuity. They 

 abound chiefly in the bark, but are found also in the alburnum 

 and even in the matured wood. They convey the descending 

 and elaborated juices. Dutrochet has vessels for this purpose 

 also, but he does not give them the same name nor structure; 

 and he has proper vessels likewise, but he does not assign to 

 them the same function. Hence there is nothing in M. Mir- 



* Rccherches Anatomiqucs, sect. i. p. 18. 



