iv.] VARIOUS THEORIES, &c. 65 



Conifers has no vessels, but consists almost entirely 

 of tracheides with bordered pits, the vast majority of 

 Dicotyledons and Monocotyledons always have vessels 

 in addition to other elements tracheides, libriform 

 fibres, and wood-parenchyma. The proportions of 

 the latter vary, and one or more kinds may be 

 absent. 



Less unanimity appeared as to the properties and 

 functions of the elements of the wood, and in fact 

 the whole controversy at one time turned upon this 

 question. It had been discovered that, broadly speak- 

 ing, the vessels contained less water and more air in 

 the summer than in the winter, and that even in 

 the tracheides there was usually if not always some 

 air. Whether the vessels and tracheides were ever 

 entirely devoid of either air or water was a disputed 

 point. Much discussion was also still abroad as to 

 the ultimate structure of the cell-walls of the elements, 

 but it was common knowledge that, whereas the 

 wood-parenchyma usually still retains its cellulose 

 walls, protoplasmic contents, starch, &c. (and the same 

 with the cells of the medullary rays), the walls of the 

 vessels, tracheides and fibres are lignified more or less 

 completely, and soon lose their living contents. 



Since most of the other points of importance to 

 the controversy were either disputed or imperfectly 



F 



