.39() Prof. H. Mohl on the Structure of Dotted Vessels. 



cavity which gives rise to the border contain air, and supposes 

 the fissure which runs above it to be at length rounded off" 

 by the further deposition of formative matter. A dependence 

 of the formation of the walls of the vessel on the condition of 

 the neighbouring elementary organs is altogether denied by 

 Meyen. 



After this exposition of the more important results of the 

 more recent investigation of the anatomy of dotted vessels, 

 I turn to my own later inquiries, and the first point which 

 demands notice is the fact, that in the dotted vessels of most 

 plants the individual tubes do not possess a uniform struc- 

 ture all round, but that their walls, according as they are in 

 contact with different elementary organs, exhibit not unim- 

 portant modifications of structure. That such a relation be- 

 tween the dotted tubes and the neighbouring elementary 

 organs does exist, is indicated by the two isolated cases cited 

 above, observed by Treviranus and Moldenhauer ; I have 

 lately endeavoured to show that this appearance is more ge- 

 nerally exhibited. I have shown that the structure observed 

 by Moldenhauer in the lime is found in other plants, as in 

 the maple [Feld-ahorn), and that the medullary rays in many 

 plants exercise a powerful influence on the structure of the 

 walls of the dotted tubes, as in those parts of the vessel which 

 are in contact with the medullary rays, the dots have an irre- 

 gular form, are surrounded by no border, are always situated 

 in those places only to which a neighbouring cell is closely 

 pressed, but never where the lateral walls of a neighbouring- 

 cell stand perpendicular to them ; that moreover the dots of 

 two vessels applied immediately the one to the other exactly 

 correspond. These circumstances, as also the frequently and 

 easily observed phienomenon, that the dots of contiguous cells 

 answer to one another in respect of situation and form, incon- 

 testably prove that the organization of the secondary coats of 

 the elementary organs of vegetables stands in close connexion 

 with that of the secondary coats of contiguous elementary or- 

 gans. 



The truth of this position has been much contested, and 

 Meyen especially, ' Physiol.,' vol. i. p. 157, denied that a proof 

 of it could be derived from the structure of the dotted tubes, 

 for he believed the appearances observed by me were analo- 

 gous to the circumstance, that in the greater part of Coniferoe 

 the lateral walls only of their tubes are studded with dots, 

 but not those turned towards the bark and pith. A surprising 

 argument surely, for the dotted tubes of Coniferce show with 

 the greatest certainty the influence which the contact of dif- 

 ferent organs has upon the organization of a third ; so that. 



