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



F. Collective vessels covered with bordered dots : the larger 

 have smooth walls ; in the less, spiral threads run between the 

 rows, Morus alba, Ulmus campestris, Clematis Vitalba. 



G. Collective vessels closely dotted : between the rows of 

 dots lie small threads, Hakea oleifolia. 



H. The larger vessels dotted, the less without dots; the 

 walls of both furnished within with spiral threads. Daphne 

 Mezereum (PL VIII. fig. 7, 8), Passerina filiforrnis, Bupleu- 

 rum arborescens, Genista canariensis. 



I. Walls of vessels which abut on other vessels dotted ; 

 those contiguous to cells with distant dots or entirely free 

 from them ; walls of either kind of vessels furnished with 

 threads. Samara pentandra, Tilia parvifolia (PL VIII. fig. 6), 

 JEsculus Hippocastanum, Acer Pseudo-platanus, Cornus alba. 

 Ilex Aquifolium, Cratcegus oxyacantha, Prunus Padus, P. 

 virginiana. 



If we take a glance at what has been said, it is clear from 

 the facts alleged, that the uniformity of the structure of dotted 

 vessels, asserted by phytotomists, exists only in comparatively 

 rare cases ; moreover, that the single point in which dotted 

 vessels agree (and even here we must pass unnoticed the 

 smaller vessels mentioned under H.), and by which they are 

 distinguished from other vascular forms, is the presence of 

 dots surrounded by a border, and which lie at least on those 

 sides which are in contact with other vessels. 



Under these circumstances then the question arises whether 

 all the alleged vascular forms are to be regarded as dotted 

 vessels, or those vessels only which on all sides exhibit bor- 

 dered dots, reckoning the rest as mixed vessels, or whether 

 new divisions of vessels are to be grounded on these differ- 

 ences. 



In my opinion the first is the proper course. All these 

 vessels possess a common character in the structure of their 

 bordered dots, by which they are easily and surely distin- 

 guished from other vessels ; and on the other hand, consi- 

 dering them collectively, the presence of dots coincides with 

 the peculiar condition of the embryo of Dicotyledons. If, on 

 the contrary, we regard as mixed vessels all those whose walls 

 exhibit an abnormal structure, our notions, so far from gain- 

 ing, would only lose in precision. Under the name of mixed 

 vessels, those vessels are usually understood whose different 

 tubes, placed in a line one above the other, exhibit a different 

 structure ; e. g. they pass from the scalary form into that of 

 annular or spiral vessels. So far as a distinct rule exists in 

 this alternation of vascular forms in many plants, especially 

 in Monocotyledons, the proposal of mixed vessels as an ex- 



