Prof. H. Mohl on the Structure of Dotted Vessels. 405 



pearance of these cavities the first trace of the dot over each 

 of them is indicated by a hghter circle, and now, by means of 

 the further thickening of the walls the formation of the vessel 

 speedily arrives at its extreme limit, at which time also the 

 transverse dissepiments are absorbed. I have not observed 

 in these vessels more than in the secondary membranes of 

 cells, any origination of secondary coats from the inosculation 

 of spiral threads. 



It should seem from what has been adduced above as to 

 the form of the canals of the dots, — whence it clearly appears 

 that the apertures of the secondary coat are so much larger, 

 and in particular so much the more di'awn out in the direc- 

 tion of their major axis into the form of fissures, the nearer 

 they lie to the centre of the tube, — that the different secondary 

 coats of the same vascular tube do not accurately agree in their 

 form. In some plants, as in Bombax pentandrum (PI. VII. 

 fig. 12, 14.), this circumstance is indicated merely by a shght 

 conical enlargement of the canal from the outer to the inner 

 side. It is far more remarkable in the form which I have 

 represented from Cassyta glabella (PI. VII. fig. 1, 4). The 

 difference between the outer and the inner secondary coats 

 attains a far more noticeable degree in Laurus Sassafras (PI. 

 VII. fig. 5), Aleiirites triloba (PI. VII. fig. 6, 8), Elcsagnus acu- 

 minata (PI. VII. fig. 10, 11), Clematis Vitalba (PI. VIII. fig. 4). 

 Here the cavities of the outer secondaiy coat present a dot 

 which is shorter than the border, while, on the contrary, those 

 of the inner coats (PI. VIII. fig. 4, 6) are extended into such 

 long fissures, that they are not merely longer than the subja- 

 cent border, but frequently run one into another and com- 

 prise the canals of many dots. These inner layers, therefore, 

 represent skins, which are imperfectly divided into broad 

 threads by long and short fissures. It is to be remarked 

 here, that the direction of the fissures of the inner layers does 

 not always perfectly agree with the direction of the major axis 

 of the canals of the dots, but intersects it at a small angle 

 (PI. VII. fig. 6). This will be the less surprising if we remem- 

 ber that the threads in Taxus which form the innermost layer 

 of the vessels run sometimes in an opposite direction to the 

 spiral line in which the major axes of the dots lie ; and that 

 the bast-cells of Apocynea are composed of coats whose sjjiral 

 striae exhibit equally a different direction of volution. We 

 find the greatest degree of difference between the outer and 

 inner coats ot the vessels in Tilia (PI. VIII. fig. 6), Daphne 

 (PI. VIII. fig. 7), and other plants adduced above under F — I, 

 in which a perfect division of the inner membrane of the 

 vessel into spiral threads exists ; a formation which clearly 



