THE LEAF 



20() 



with the xylem of the fibrovascular bundle. Before we consider 

 the nature of this relation it will be well to examine more carefully 

 the bundle itself. The conductive strand is represented by its 

 xylem only; this consists, interestingly enough, of two regions 

 a lower one in contact with the empty space once occupied by the 

 phloem, and an upper, consisting of distinct rows separated by 

 thinner-walled elements. The latter is the true centripetal or 











FIG. 156. Longitudinal section of the leaf of Prepinus statenensis. Explanation 

 in the text. 



cryptogamic wood which in Prepinus alone among the conifers is 

 present in a typical form. The lower region of the xylem is cen- 

 trifugal and corresponds to the whole of the xylem in the leaf of 

 the modern Pinus. In the modern or centrifugal region of the 

 wood, rays are distinctly present, and these pass from its substance 

 into the cavity once filled by the phloem. The centrifugal region 

 of the wood provided with rays is related to the inner transfusion 

 sheath on its flanks alone. This relationship corresponds in fact 

 to that observed in the case of living pines, as is shown above in 



