40 



such perfect instruments ; for most of the incorrect observa- 

 tions of former times can only be ascribed to the defective mi- 

 croscopes of that period. 



M. Mohl compares the development of the porous vessels 

 with that of cells, as series of thin-sided cell-like cavities con- 

 stitute their base, in which the spiral fibres are then formed. 

 M. de Mirbel had already started a similar notion, that vessels 

 are formed from cells, and the observation of the porous tubes 

 in the earliest stages of their development is said to prove this. 

 About this time the individual cells are frequently found per- 

 fectly closed, and the thin membranous diagonal partitions 

 subsequently disappear, while they remain in many cases 

 during the whole lifetime of the plant, but take a structure 

 quite different from that of the lateral partitions, which has 

 already been demonstrated in various plants. I am well ac- 

 quainted with the cases which might lead observers to the 

 above views, but I also know of numerous cases in which the 

 very opposite may be observed, where both the simple as well 

 as the metamorphosed continuous spiral tubes separate in the 

 course of development more or less completely by constric- 

 tion, and form articulations arranged in series. 



The cross partitions of the single articulations of the meta- 

 morphosed spiral tubes are either broken through by a very 

 large aperture or by a number of fissures and longitudinal 

 pores : nay, even the oblique partitions of the large dotted 

 tubes in the wood of some species of Ephedra are pierced, 

 and that by the large round holes, which generally occur in 

 them arranged in two parallel rows. M. Mohl observes that 

 phytotomists have considered these horizontal sides as lateral 

 sides of the tubes, which was also probably owing to bad 

 instruments, for the inclination of these horizontal walls to 

 the lateral is so exceedingly small, that they may even be re- 

 garded as inclined terminal surfaces of the prosenchymatous 

 cells, with which the superposed cells stand in connexion ; 

 I at least have declared in favour of this latter opinion. The 

 disappearance of the horizontal walls in the dotted spiral 

 tubes is, as M. Mohl thinks, to be compared to the formation 

 of the vessels of the latex, which also are said to originate from 

 cells standing above one another, as M. Unger (see p. 30) has 

 tried to render probable by a drawing. The most varied views 



