398 Prof. H. Mohl on the Structure of Dotted Vessels. 



The firmer establishment of what has been asserted will be 

 found in the facts which I shall now detail. 



If we examine the organization of dotted vessels with re- 

 ference to their circumference, we find that in a proportion- 

 ally small number only of Dicotyledons they possess a struc- 

 ture independent of the surrounding organs. Hence such 

 vessels only come naturally under observation in which, on 

 accurate inquiry, one is really convinced that they do stand 

 in contact with elementary organs of a different nature, and 

 therefore all vessels must be excluded which are surrounded 

 only by prosenchymatous or parenchymatous cells, since these 

 always have on every side uniform walls. If we observe ves- 

 sels running along unconnected with such cells, as e.g. is 

 normally the case in Rhamnus capensis, Viburnum Opulus, we 

 find a series of modifications of vascular structure in which 

 the influence of the contiguous organs is in general exhibited 

 with great distinctness. 



A. The peculiar structure of the dotted vessels is most, 

 perfectly developed in those plants in which the walls of the 

 vessels exhibit no variations, whether standing in contact with 

 other vessels or with cells, in which they are therefore studded 

 uniformly with dots which are surrounded by a border, as 

 in El&agnus acuminata, Clematis litalba, Broussonetia papy- 

 rifera. 



B. To these vessels succeeds a second form, in which those 

 sides of the vessels which stand in contact with prosenchy- 

 matous cells are in like manner furnished with equal dots 

 surrounded by a border, but in which the influence exercised 

 by the neighbouring cells is declared by the fact that the dots 

 on the walls which abut on the cells are placed at greater 

 distances. Such vessels occur in Bioca Orellana, Acacia lo- 

 phantha, Sophora Japonica. 



C. With a stronger, more decided dependence of the ves- 

 sels on the cells, the walls abutting on other vessels remain 



of the spiral, and, therefore, if the spirals in both are homodromous 1 , they cross 

 one another. The more indistinct the spiral structure of the secondary mem- 

 brane is, and the more it approaches to a reticulate form, the more visible 

 is the dependence of the secondary coats of one organ on those of another. 

 The dots of both organs now correspond not only in position, but also in form 

 and in the direction of their major axis, as for instance in scalary tubes ; 

 therefore they no longer cross with those of the contiguous organ ; they no 

 longer run over the angles of their own organ, but are closed in their neigh- 

 bourhood, and are influenced in length by the size of the lateral facets of the 

 contiguous organ. On this depends the difference which we find between 

 the lateral walls of scalary tubes, according as they abut on a vessel or 

 cell, &c. 



' Compare Ann. and Mag. Nat. Hist., vol. viii. p. If), and note. — Ed. 



