M. Mohl on the Structure of Annular Vessels. 19 



cided separation into isolated threads, from deviations in the 

 course of these from that of the main-thread, or from a reticu- 

 lated union of the isolated threads. 



The direction in which the spiral fibre is wound has, indeed, 

 no direct connexion with the organization of the vessel ; never- 

 theless I think it necessary to make some remarks on this sub- 

 ject, because several erroneous statements, partly founded upon 

 an imperfect knowledge of the spiral, have been advanced by 

 some authors. I have elsewhere said that the great majority 

 of spiral vessels were wound to the right ; that is, the volution 

 of the fibre is such, that, to an observer placed in the axis of 

 the cylinder around which the spiral line rises, the fibre ap- 

 pears to mount from left to right, as shown in the vessel repre- 

 sented at figure 5. Like most other phytotomists, Schleiden 

 says that the spiral fibre is wound sometimes to the right, 

 sometimes to the left ; and he thinks it possible to admit pro- 

 visionally, as a general rule, that in spiral organizations (i co- 

 temporaneously" developed, those which are situated imme- 

 diately side by side in the direction of the radius have similar 

 directions (homodromes), whilst those placed side by side in 

 directions parallel to the periphery have different directions 

 (heterodromes) ; and in proof of this law he relies on the con- 

 stant crossing of the pore-like fissures in contiguous parenchy- 

 matous and ligneous cellules when observed in sections made 

 parallel to the medullary rays. I must confess my inability to 

 conceive how Dr. Schleiden can allege the crossing of the porous 

 fissures in support of the volution of the fibres in different di- 

 rections, since it proves quite the contrary. This crossing is 

 seen when two vessels or cellules furnished with pores are su- 

 perposed, and the adjacent parietes wound in opposite direc- 

 tions ; but it is evident that this last case is only possible when 

 the winding in the two vessels is homodromous *. It is quite 

 true that we generally see the porous fissures crossing each 

 other in a section parallel with the medullary rays ; which 

 proves that the different layers of cells visible in such a section 

 and placed one under the other, are wound in a homodromous 

 direction ; but as at the same time the cells of every such 

 layer are homodromous with each other, it clearly follows that, 

 generally speaking, all the cells of a plant are homodromous ; 

 and this, in fact, will be found to be so on an examination of 

 different sections of the same plant. 



Without doubt spiral vessels exist which are wound to the 

 left; but although I have latterly found them more fre- 

 quently than formerly, I must still persist in asserting them 



* [This illustration may be easily verified by applying two quills toge- 

 ther, with equidistant homodromous or heterodromous spirals scratched upon 

 them.— En.] 



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