28 



DISEASES OF TREES 



it is evident that they elongate not only towards the side next 

 the wood but also towards the side next the cortex. With 

 the formation of new phloem-tissues the cortex-roots are con- 

 stantly being pushed away from the cambium mantle, as may be 

 seen from Fig. 3. In the case of such trees as the silver fir, 



whose stem remains smooth for many 

 decades before the formation of true 

 bark begins, the cortex-roots may 

 thus be pushed away from the cam- 

 bium mantle without their suffering 

 any appreciable injury. They may 

 attain to an age of forty years, a 

 corresponding age being reached by 

 the sinkers, whose length increases 

 in proportion to their age. On the 

 other hand, trees like the Scotch pine, 

 which form bark early, show only 

 short sinkers, of a length of 3-4 cm., 

 and an age of twelve to fifteen years. 

 This is to be explained by the fact 

 that owing to the usually more active 

 formation of new bast the cortex- 

 roots are more quickly pushed away 

 from the cambium mantle. The outer 

 parts of the cortex are converted 

 into bark, and as soon as a portion 

 of cortex containing a cortex-root 

 is converted into bark, it dries up, 

 along with that part of the mistletoe 

 root which it contains, and the con- 

 nection with the sinker is broken. 

 This is distinctly brought out in 

 Fig. 3. The sinker then ceases to 

 grow, and is covered over sooner 



or later by the new wood-rings. Of course the death of a cortex- 

 root does not take place simultaneously throughout its whole 

 length. On the contrary, the oldest part that is to say, the part 

 situated farthest from the cambium dies first, whereas those 

 younger portions which are still enveloped in living cortex 



FIG. 3. Cross section of a- stem 

 of Abies pectinata containing 

 Visciim album, a, dead bark 

 showing dead cortex-roots ; b, 

 region of living bast ; f, cam- 

 bium region ; d, cross section 

 of a cortex-root showing a 

 sinker six years old ; e, a 

 sinker eighteen years old ; the 

 cortex-root has lately been 

 enveloped in the bark, while 

 the apex of the sinker has 

 withered in the duramen ; at 

 / the cortex-root and the por- 

 tion of the sinker in the bast 

 have been dead for two years ; 

 a cortex-root is shown at g 

 which has been dead for six 

 years. The boundary line 

 between the duramen and al- 

 burnum lies at hh ; at x two 

 sinkers are shown, those por- 

 tions situated in the alburnum 

 being still alive. 



