Prof. H. Mohl on the Structure of Dotted Vessels. 403 



fissures which in Chilianthus arboreus clothe the walls of the 

 vessels which abut on other vessels (PI. VIII. fig. 2), and give 

 them the appearance of scalary tubes. Each of these fissures 

 is surrounded by a border which depends upon the existence 

 of a hnear cavity running beneath the fissure, which is much 

 wider than it, as is plainly observable in vertical sections of 

 contiguous vessels (PI. VIII. fig. 3). They are distinguished 

 at first sight from ordinary scalary tubes, though so similar in 

 other respects, by the presence of this cavity, of which no 

 trace exists in scalary vessels, as we may convince ourselves 

 in tree-ferns and large Monocotyledons. 



Besides these bordered dots which distinguish dotted from 

 other vessels, as we have seen above, there is, in a great 

 number of plants, yet another modification of dots which are 

 surrounded by no border. These are most frequently found 

 in those situations which lie near to medullary rays ; there are 

 however also vessels in which all the walls not in contact with 

 another vessel exhibit this form of dots, e. g, Cassyta, Bom- 

 bay pentandrum (PL VII. fig. 13), Hernandia ovigera, Chili- 

 anthus arboreus (PI. VIII. fig. 1). These dots have generally 

 a far less remarkable size than those which are bordered, and 

 generally a transversely oval form. On a more accurate ex- 

 amination they are found to be surrounded by a double line, 

 so that in many cases (especially in Aleurites triloba (PL VII. 

 fig. 7)) one might almost be forced to ascribe to them also a 

 small border. A more perfect inspection, especially of anx 

 oblique section of the walls of these vessels, shows, on the con- 

 trary, that between these dots and the neighbouring organs 

 no cavity exists, but that they are formed by a simple per- 

 foration of the secondary coat, and therefore accord perfectly 

 with the dots of parenchymatous cells, reticulate vessels and 

 scalary tubes. The double line surrounding the orifice of the 

 dots arises from their being generally somewhat wider towards 

 the inside of the vessel ; and therefore, if we examine them in 

 a direction perpendicular to the wall of the vessel, both the 

 inner orifice of the canal of the dot in the inside of the ves- 

 sel, and the outer closed by the primary coat, are at once 

 visible. If, as is not rarely the case, the canal perforates the 

 wall of the vessel in a somewhat oblique direction, if we look 

 down perpendicularly upon the walls, both these lines co- 

 incide on the one side of the dot, or cross each other (PL VIII. 

 fig. 5 a a, Cactus brasiliensis). 



It is clear, that the dotted vessels, by means of this last kind 

 of dots, form the transition to scalary and reticulate vessels, 

 as they exist in vascular Cryptogams and Monocotyledons, 

 for the walls possessing these dots agree perfectly with the 



