DISEASES OF TREES 



the part of the branch that is not completely lignified and 

 always exactly parallel to the longitudinal course of the xylem 

 elements. The apex of the root, which is flat upon its inner 

 side, advances in a definite region of the young wood, at the 

 same time pressing out, splitting, and dissolving the still 

 unlignified elements by means of its con- 

 vex outer surface. This goes on till future 

 progress in the original direction is pre- 

 vented by the resistance consequent on 

 lignification in the outer layers of the new 

 wood which the roots of the parasite are 

 unable to split off and dissolve. The root, 

 whose apex is thus in a cul-de-sac^ is forced 

 to form a new growing-point some distance 

 behind the apex namely, atthe place where 

 the convex outer side comes into contact 

 with the cambium zone (Fig. 7, y). From 

 this new point growth in length begins 

 afresh, and is continued in a zone situated 

 nearer the periphery of the wood. During 

 the development of an annual ring the root 

 FIG. 7. Youngest por- of Loranthus (which, of course, can only 



tion of a root of L. grow Curing the period when the cam- 



europ&us. a, cortex 



and bast ; b, cam- bium produces young wood) is generally 



thrice compelled to shift the direction of 

 wood-ring of the cur- its growth farther out, the result being 



rent year in which , , . . _ 



growth has been com- that the inner side of the root shows a 

 pleted ; e, wood-ring corresponding step-like arrangement, which 



of the previous year ; * 



z, root of Loranthus ; accords with the advance of the wood, as 



J\t a p e oint f wh e r r e 0t r; is shown in Fi S s " 6 and 7- The distance 

 new root-apex is form- between two steps measures from 5 to 8 



mm., while the annual growth in length 



of the root amounts to about 1*5 cm. As the roots grow in a 

 direction opposed to that pursued by the ascending water, the 

 latter flows directly from the conducting elements of the wood 

 into the roots of the Loranthus, at the points of depression. 

 The root possesses the power of growing vigorously in thick- 

 ness, whereby it is enabled for a series of years to keep pace 

 with the increase in thickness of the oak-branch, and thus 



