Prof. H. Mobl oil the Structure of Dotted Vessels. 399 



very thickly studded with dots ; only those in contact with 

 prosenchvmatous cells are set with very remote dots, or even 

 (at least for considerable distances) quite free from them. 

 The portions bordering on the medullary rays have simple 

 dots. Such vessels occur in Sambuciis nigra, Betula alba, 

 Aralia spinosa, Corylus Avellana, Populus alba, Ahius incana, 

 Platanus occidentalis, Pyrus mains, G-ymnocladus Canadensis. 



D. With a yet stronger influence of the contiguous cells, 

 Avhich possess more commonly the form of parenchymatous 

 than prosenchymatous cells, those portions of the walls only 

 vhich abut on other vessels exhibit dots surrounded by a 

 border ; those portions, on the contrary', abutting on cells, have 

 frequent and large perfectly borderless dots, altogether resem- 

 bling those of the parenchymatous cells, e. g. Cassyta glabellay 

 C. filiformis, Bombax pentandrum (PL YII. tig. 12, 13), Her- 

 nandia ovigera. 



E. We have a mere modification of this structure, though 

 possessing a very peculiar appearance, in the form in which 

 the walls which abut on another vessel are fashioned like sca- 

 lary vessels (PI. VIII. fig. 2, from Chilianthus arbor eus), in 

 consequence of the dots being drawn out into fissures which 

 extend the whole breadth of the vessel, while the walls which 

 are contiguous to cells are studded with large unbordered 

 dots (PI. VIII. fig. 1.). This form is beautifully developed in 

 Chilianthus arhoreus and Cynanchum obtusifolium. In a less 

 degree the same phenomena are exhibited by Vitis vinifera 

 in the walls contiguous to vessels. 



The greater number of dotted vessels can be referred to 

 one of the heads just enumerated. We have however now to 

 examine in addition a series of vascular forms which agree 

 in the intervals between the rows of dots not being smooth, 

 but marked on the interior wall with a spiral line. 



These vessels are to the ordinary dotted vessels what the 

 dotted tubes of Taxus are to the other Conifer oe. In these 

 vessels not only similar variations occur, as regards the dis- 

 tribution of the dots, as in the lately enumerated vascular 

 forms, but other differences occur, according as a part or all 

 the vessels possess these spiral threads. In some of these 

 plants we may, for instance, though not very nicely, distin- 

 guish greater and less vessels of a not always similar structm'e ; 

 their vessels form groups, especially in the inner part of the 

 annular rings, and near these groups, which consist of large 

 vessels, lie others of a far less calibre, whose tubes approach 

 more to the form of prosenchymatous cells, and which I shall 

 indicate in what follows by the name of little vessels. 



These vessels may be arranged under the following heads : 



