38 Dr. W. C. Williamson on 



contained within each " basis of bifurcation " are reduced to 

 one-half in each of the younger branches, and that a similar 

 diminution occurs in the leaves with which the cortex is 

 clothed. It must be remembered that such dichotomies 

 can only occur when the twigs about to be divided are 

 terminal ones, provided with a growing point. 



But, besides these equal dichotomies, there exists in 

 these plants a series ot unequal ones of a different character, 

 and leading to different results. These present themselves 

 in two secondary forms. In one, instead of the cylinder of 

 Primary Xylem being divided into two practically equal 

 halves, a very small segment of the circle is cut out of it, 

 and passes outwards as a branch, carrying with it a small 

 portion of the medulla. As it does so this unsymmetrical 

 segment gradually changes its form, becoming a solid 

 tracheal strand without any trace of a medulla ; but 

 occasionally we find that a similar bundle retains a cylin- 

 drical medulla in its interior. This latter condition is rare ; 

 but in most cases, on reaching the axis of the strobilus 

 it is intended to support, a central medulla is developed. 

 The solid type just described seems to be confined 

 to strands destined to supply organs of fructification. 

 In the second form of this unequal dichotomy, instead 

 of the segmentation reaching the medulla, a limited 

 number of tracheids is detached from the periphery of 

 the Primary Xylem strand, the segmentation not reaching 

 the medulla. Strands thus produced also go to reproductive 

 organs. 



My renewed researches have been made among 

 numerous types, from which I have selected the following, 

 because they supply me with a large range of specimens of 

 various dimensions and, as I believe, of different ages, 

 calculated to throw light upon some of the debated 

 questions : — 



