Prof. H. Mohl on the Structure of Dotted Vessels. 401 



press division is fair enough. But if we would regard as 

 mixed vessels those in w r hich the different sides exhibit a dif- 

 ferent structure, we confound two things which possess no- 

 thing in common, as in the former case the structure of the 

 vessels depends on the course of the vascular bundle, in the 

 latter on the structure of the contiguous elementary organs. 



In my opinion nothing could be worse than to have re- 

 course to different names for every slight modification of the 

 dotted vessels. Some modern phytotomists have, alas ! taken 

 this path with respect to cellular tissue, a path which would 

 soon lead to the same lamentable labyrinth of terminology in 

 the anatomy of plants as perplexes us in systematic botany. 



Most phytotomists make the difference between dotted and 

 scalary vessels to consist in the presence of many small dots. 

 But magnitude and number are far too relative notions upon 

 which to ground an accurate division; we must therefore 

 search for some better distinctive marks. Kieser believed 

 the transverse bands (Querbander) to be characteristic of the 

 dotted vessels as well as the dots ; but that such is not the 

 case, inasmuch as these bands are merely the limits of the suc- 

 cessive tubes, and also occur in other vascular forms, is clear 

 from the satisfactory inquiries of Moldenhauer and others. 



Since these characters also are of no value, we must look 

 for the distinctive sign of the dotted vessels in the structure 

 of the dots themselves, and especially in the circumstance 

 that either all the dots, or at least those which lie on the walls 

 abutting on another vessel, are surrounded by a border. 



I should esteem it superfluous, after what I have brought 

 forward in my earlier labours on the structure of bordered dots, 

 to recur to this point if some interest did not appear to be at- 

 tached to an enumeration of the modifications which are found 

 in the dots of different plants. 



Of all the plants I have examined, none appears so fit for 

 acquiring a knowledge of the structure of these organs as 

 Cassyta glabella, on account of the remarkable size of its dots 

 (PI. VII. fig. 1). In this plant, if we take delicate transverse 

 or longitudinal slices (PL VII. fig. 2), we can with the great- 

 est certainty be convinced that the border of the dots depends 

 on a cavity (a) which lies between the contiguous walls, and 

 that the dot itself (b) is a canal running from the inside of 

 the vessel to this hollow, and at its outer extremity closed 

 with a delicate membrane. It is rather more difficult to re- 

 cognise the structure in other plants, yet in those whose dots 

 are not too small, as in Laurus nobilis (PL VII. fig. 9), L. Sas- 

 safras, Aleurites triloba, Acacia lophantha, it is quite possible 

 with the help of a good microscope to do so. 



