32 



THE LARCH CANKER 



found a similar formation of resin ducts in larch which has 

 been attacked by the sawfly. These comparisons do not 

 support the contention of de Vries (1876) that such wood 

 is the result of reduced tangential pressure produced by 

 a wound, since in two cases the tree was not wounded 



FIG. 11. Radial longitudinal section of abnormal wood: a, summer 

 wood of previous year, abnormal one cell shows tertiary thickening ; 

 &, short sclerenchymatous elements ; c, long sclerenchymatous elements ; 

 d, nearly norma-1 xylem. 



except in its crown. We have rather to look for the cause 

 in the reduction of 'food supply which may be brought about, 

 either by defoliation, or by the death of the phloem in the 

 neighbourhood of the developing wood. 



In the canker the formation of normal wood is resumed 

 only when new ])hl<>om lias been made outside, which forms 



