454 Dr. Willshire on some points of Vegetable Structure, 



may all preserve the same direction, save in a very few spots, 

 the curves fitting into each other; no intervals being formed 

 betvi-een them, but one continuous layer resulting from their 

 grovith and approximation, except in the few places just al- 

 luded to (PI. XII. fig. d.) : such appears to be the case in ves- 

 sels whose punctations are few and scattered. Now it is in 

 those spaces which result from the opposition of the smaller 

 curves (fig. c. [b)) that the punctations are formed, nothing 

 there existing but a layer of external membrane, which be- 

 comes depressed in the form of a hollow or excavation towards 

 the centre of the tube, the edge of the depression being the 

 opposed curves (fig. c. d. and e,). This answers to the larger 

 surrounding circle of the dot, der Hofdes Tupfels of the Ger- 

 mans. 



According to the size of the curves so will be that of the 

 circle of punctation, and according to the shorter or more 

 elongated spiral direction of the sinuous fibres along the pri- 

 mary layer, so will be the position of the circles with respect to 

 each other. Thus in the tubes of many Coniferce the punc- 

 tations are large, and placed in a single row^ down those walls 

 of the vessel which are in approximation to others, whilst 

 those parietes in juxtaposition with true cells have small cir- 

 cles only, and often distant from each other (fig. e.). In these 

 cases the spiral direction of the fibre is very elongated, and 

 opposition of curves, the latter being large, ensues in a limited 

 manner and apparently overruled by the nature of the adja- 

 cent organs, which fully establishes one part of MohFs theory, 

 namely, that contiguous structure influences the formation of 

 the punctations. In the Coniferce, from many of the curves 

 being similar in direction, there is much fibre consolidated 

 into apparent homogeneous substance (fig. e, one extremity is 

 drawn homogeneous from the consolidation of the fibres, which 

 is the natural appearance of the whole tube, save where the 

 punctations exist ; the position of these indicate the direction 

 and curves of the fibres, though not actually apparent). With 

 respect to the dot seen in the circle of depression, MohPs view 

 appears to us to be correct, that it is a canal traversing the 

 walls of the vessel, thickened by superimposed matter from 

 the interior of the vessel to the bottom of the excavation : 

 that the external point of this canal is not pervious, is also 

 probable from the appearance it presents (fig. g.). 



We must differ from Mohl in looking upon many of those 

 instances which he adduces as examples of the dot without 

 the circle or hollow, as instances of small hollows or depres- 

 sions without the dot : in these cases, it seems to us, the great 

 mass of spiral fibres has curves agreeing in direction with 



