159 



difficult to conceive, that if a vessel, composed of many spiral fibres, 

 which are a little remote from each other, have the several turns of 

 the fibres connected by longitudinal ones, it will present a surface 

 composed, like a net, of quadrangular meshes ; this is, in fact, the 

 case. The vessels, whose structure has been detailed, are in pro- 

 gress towards the dotted condition ; some, undoubtedly, from the 

 growth or elements of nutrition being suspended, proceed no farther 

 than the state in which they have been described, but others arrive at 

 their perfect development, the angles of the meshes being rounded, 

 and the dot becoming more or less oval ; they then appear to be dot- 

 ted vessels of a certain kind. In some instances the various steps 

 of the progress can be witnessed in the same vessel in Carina bicolor. 



In examining vessels of this plant, it is very apparent that the depo- 

 sition of the longitudinal fibre, from the early contents of the vessel, 

 is a process far subsequent to the formation of the spiral fibre, as 

 many vessels will afford the opportunity of seeing the latter perfectly 

 formed, and the former absent, or only commencing to be formed, at 

 one or the other extremity. 



These vessels point out to us some of the means by which Na- 

 ture, having first established laws, can, with the simplest elements, 

 afterwards effect complicated changes, still maintaining the same 

 unity of design in one plant, or portion of a plant, as in another. 



When other kinds of dotted vessels, such as occur in woody exogens, 

 are the subjects of examination, it will be found that the dots on such 

 are of a still more complex nature, and which, by the investigations of 

 the indefatigable Mohl, have been clearly elucidated. Tn all vessels 

 of this kind the dots possess a central smaller dot, which appearance 

 is chiefly due to a small depression occurring between two adjoining 

 vessels, and projecting from without inwards in each vessel at the same 

 spots, and to a deposit of "sclerogen," or fibrous matter in the in- 

 terior of the vessel, which is thicker in every part than on the sum- 

 mit of the projections ; in this way presenting to the eye a disk, with 

 a centre of green or reddish colour, arising from the tenuity of the 

 membrane decomposing the light in its transit through such a minute 

 space. 



