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



walls of scalary vessels which abut on parenchymatous cells. 

 The relation of these two forms of vessels is also indicated by 

 the fact, that in particular Dicotyledons the dotted vessels are 

 altogether replaced by reticulate vessels (V\.Y\\\.fi^.o, Cactus 

 brasiliensis). As regards the transverse dissepiments of dotted 

 vessels, I confine myself to a few remarks, as I have before 

 proved that the dissepiments of vascular tubes are not always, 

 though the case is generally otherwise, absorbed in the course 

 of the formation of vessels, but sometimes remain, though in 

 that case pierced with real perforations. In the dotted ves- 

 sels these dissepiments appear chiefly under two forms : 

 either the original dissepiment in great part remains, and a 

 round aperture is formed in its centre, whose diameter is 

 about one-half or one-third of that of the dissepiment ; e. y. 

 Cassyta glabella (PI. VII. fig. 3), Ficus martinicensis, Cactus 

 brasiliensis (PI. VIII. fig. 5); or they are perforated with many 

 transverse fissures seated one above the other so as to re- 

 semble the walls of a scalary vessel. 



This last form I find only in the obliquely lying dissepi- 

 ments ; they exist, e. (j. in Betula alba, Fagus sylvatica, Cory- 

 lus Avellana, Alnus incana, Platanus occidentalis. Viburnum 

 Ojmlus, Ilea- Aqwfoliimi ; while the first form is more frequent 

 in those which are horizontal. The dissepiments of the same 

 plant do not however always exhibit the same structure, but 

 some may possess the form of a scalary wall, while others are 

 completely absorbed. Oblique dissepiments have such a di- 

 rection that their surface comes into view on a longitudinal 

 section parallel to the medullary rays. 



I shall add merely a few words on the history of the de- 

 velopment of dotted vessels. They appear in the early period 

 of their evolution, like the other vessels, as rows of large, cell- 

 like, perfectly closed tubes whose skin is thin and perfectly 

 uniform, of which every one possesses a nucleus {Zellenkern). 

 At a later period we see in the lateral walls, especially those 

 resting on other vessels, as it were a delicate fibrous net-work. 

 A further inspection of the development shows that this does 

 not (as one might at the first glance have been induced to 

 believe) depend on threads deposited on the inner walls of the 

 vessel, but that the meshes of the net correspond Mith the 

 future borders of the dots, and therefore indicate the cavities 

 which lie between the vessels ; that the apparent threads which 

 surround the meshes are formed by the places of the walls 

 of the vessel which remain in contact with the neighbour- 

 ing organ ; and that at this time, as well as during the whole 

 process of development, the vascular tubes are filled with sap, 

 and not with air as Schleiden asserted. Shortly after the ap- 



