I.] GENERAL CHARACTERS AND STRUCTURE. 9 



once begin to form the tracheides with thin walls and 

 large lumina, because the pressure previously exerted 

 by the cortical jacket has been reduced by its cracking, 

 &c. during the winter rest, and it is the sharp contrast 

 thus displayed between the newly-formed tracheides 

 with thin walls and large lumina, and the compressed 

 denser ones of the previous summer on which they 

 suddenly abut, that produces the impression of the 

 " annual ring." 



It is now time to attempt to give some clearer ideas 

 of what this " cambium " is, and how its cells become 

 developed into tracheides. But first it is necessary to 

 point out that each tracheide examined singly, is a 

 long, more or less tubular and prismatic body, with 

 bluntly tapering ends, and the walls of which have 

 certain peculiar markings and depressions on them, as 

 seen in Fig. 4. We cannot here go into the important 

 signification and functions of these markings and de- 

 pressions however, since their study will need a 

 section to themselves. It must suffice for the 

 present to state that the markings have reference 

 to the minute structure of the cell-walls, and the 

 depressions are very beautiful and complicated 

 pieces of apparatus to facilitate and direct the 

 passage of water from the cavity of one tracheide 

 to that of another, and prevent access of air. Now, the 

 cambium is a thin cylindrical sheet of cells with very 



