STRUCTURAL VARIATIONS IN HOMOLOGOUS CELLS 157 



This process apparently goes through to completion once in about 

 every two days throughout the season, and leads to the production of 

 radial files of tracheids equal in number to the cells in one tangential 

 layer in the cambial region, every cell in one file having developed from 

 one and the same cambial cell. This provides us with a remarkably 

 uniform tissue. Towards the end of the season, the radial expansion on 

 differentiation becomes less and less so that the tracheids become 

 radially thinner while retaining the same tangential dimension,* and 

 at the same time the walls become thicker. It is tracheids of this latter 

 type which we have been considering up to now. For the moment we 

 will here confine our attention to the larger tracheids in the early wood 

 for a very simple reason. It will be evident from the above discussion 

 that there might be a distinct difference between radial and tangential 

 walls in tracheids in virtue of their different associations with develop- 

 ment in the cambium; the radial walls are continually stretched laterally 

 during both differentiation and during the growth back to normal size 

 of the cambial initials after a division, while the tangential walls are 

 new at each division (and therefore in each tracheid) and do not undergo 

 much lateral extension. Now in the late wood, not only are the radial 

 walls difficuU to observe on account of their narrowness relative to the 

 tangential walls (isolated cells lying therefore on their tangential and 

 only seldom on their radial walls), but it is difficult to distinguish them 

 from tangential walls since the only difference is in width. This 

 difference cannot be used here since, as will be seen later, all the walls 

 other than the one being observed are, of necessity, removed. In the 

 spring wood tracheids, however, none of these objections apply. The 

 cells are just as liable to lie on their tangential as on their radial faces, 

 and these can be distinguished since only the latter are pitted. This 

 absence of pits on tangential walls is, in fact, rather general among 

 tracheids except in the last few cell layers of the late wood, and it is 

 this exceptional case which makes it particularly difficult to use late 

 wood tracheids for the present observations. 



Now while this process is going on, the cambial cells are continually 

 elongating. This change in dimension is much slower than the lateral 

 changes associated with the longitudinal tangential division, but never- 

 theless reaches in time very considerable proportions so that the cells 

 may double their length in some thirty years (see e.g. Table XIII). It 

 therefore follows that, since the cambial cells are continually cutting off 

 . "replicas" of themselves, then tracheids in the inner annual rings of 



* In fact becoming progressively somewhat wider, a minor change in dimension 

 which will not be considered here. 



