M. Mohl on the Structure of Annular Vessels. 23 



the greater thickness of their fibres, of the greater distance of 

 these organs from each other, and of the absence of the mu- 

 cilage with which the young vessels are gorged, these de- 

 veloped vessels present a much clearer contour, and the or- 

 ganization of their fibres is more easily observed. Doubtless 

 it is true that we ought not to infer from the structure of a 

 developed organ the mode of its development ; but the exami- 

 nation of this structure is nevertheless of very great importance 

 in studying the manner of its development, since we always 

 thence obtain the means of proving the truth of any theory 

 propounded on the history of development, a theory which 

 ought not to be in contradiction with the results of an exa- 

 mination of the developed organ. Now in the present case 

 this contradiction assuredly exists between the structure of 

 the developed annular vessels and the theory of Schleiden. 



Let us now see what information the examination of the 

 young vessels gives us of the mode of their development. At 

 first I selected the stems of different plants, especially of 

 Tradescantia tuherosa, because Schleiden announced that he 

 had remarked the metamorphosis of spiral vessels into annu- 

 lar vessels in the youngest internodes of subterranean and 

 ascending stems. The results have not been favourable to the 

 theory of Schleiden. For this examination it is not proper 

 to select vessels placed at the interior angle of the vascular 

 bundles, because these pass too rapidly through the phases of 

 their development, and their diameter is also too small ; the 

 coils of their fibres being moreover at first too close together 

 to allow any observations made upon them to be considered as 

 conclusive. The larger vessels, placed more towards the exte- 

 rior, present less difficulties in these respects, though here also 

 an unfavourable circumstance occurs, viz. that the rings in the 

 course of their development, in consequence of the feeble lon- 

 gitudinal growth of the vascular utricles, remain very close 

 together, which may, in some cases, render the distinction of 

 the annular and spiral formations in the fibres difficult, and 

 which, in all cases, makes it rather hard to decide whether 

 there does or does not exist between each pair of rings a 

 slender spiral fibre which is subsequently absorbed. However 

 I think I have observed with certainty, that from the begin- 

 ning, and so soon as I could distinguish the fibres on the in- 

 terior surface of the vascular utricle, under the form of thin, 

 more or less narrow, diaphanous edges, they were not abso- 

 lutely spiral ; but that, as in the developed vessels, they formed 

 either complete isolated rings, or rings intermixed with spiral 

 fibres ; so that, with the exception of the thinness of the fibres, 

 and of the small distance of the rings from each other, there 



