24 M. Mohl on the Structure of Annular Vessels, 



is no essential difference observable between them and the 

 perfectly developed vessels. 



The examination of the vessels of the stem not havings how- 

 ever, furnished me with a perfectly satisfactory result, and 

 my former researches on the roots of Palms and other mono- 

 cotyledonous plants having shown me the greater facility of 

 studying the development in this organ than in the trunk, I 

 submitted the roots of Tradescantm to a very attentive exa- 

 mination, the results of which I consider to be quite conclusive. 

 The examination of the roots presents this great advantage 

 over that of the stems, that in the larger vessels, placed nearer 

 to the centre, the fibres are not developed until a sufficiently 

 late period, when their longitudinal growth is already termi- 

 nated. At the period when the fibres of the vascular utri- 

 cles are developed, these utricles have not only already 

 attained to a considerable size, but the fibres in them are 

 also, from the beginning, arranged at greater distances from 

 each other, and their successive development may be fol- 

 lowed in detail step by step, from one end of the root to the 

 other. This examination is rendered easier in consequence 

 of the vessels being deposited in a very transparent cellular 

 tissue. In these researches I have recognised with the great- 

 est clearness, and with a perfect conformity to what I had 

 previously observed in the roots of Palms, that, from the time 

 when the fibres make their appearance, and when they are 

 still so tender, narrow and transparent, that it is often only 

 possible to see them with a faint light, they already present 

 all the different modifications of form which are observed in 

 the perfect vessels. We then find, as at a later period, the 

 same alternation of annular and spiral and reticulated fibres ; 

 but I have never seen the least trace of the formation in all 

 vascular utricles of a spiral fibre whose coils would unite in 

 pairs, and the portions of the spiral fibre serving as the means 

 of union be absorbed ; and I consider it as perfectly impossible 

 that this transition of spiral vessels into annular vessels, if it 

 existed, could have escaped me, because in a great number 

 of roots 1 have followed the vessels from the moment when 

 the utricles presented closed cells with thin parietes_, and en- 

 closed a nucleus. 



Hence it results that the development of the annular ves- 

 sels agrees with the observations made on the perfect vessels. 

 Researches into these two organs show that annular, spiral, 

 and reticulated vessels afford three different forms, very inti- 

 mately connected, and passing frequently one into the other ; 

 but that they must not be considered as temporary degrees of 

 metamorphosis of the same vascular utricle. It is true that 



