6oo 



In Fi<,^ 141 B, which gives a cross-section of the canker knot (k) from 

 I'ig. 141 ./, ;;; indicates the pith body; a, the uninjured annual ring of the 

 first year's growth; b, the cleft ring of the second year; c, the wood of the 

 third year, which is growing out into the canker swelling {k) ; i represents 

 the firmer tissue islands and stripes in the tinder-like ground tissue. 



In the cases which lia\c been obser\ed up to the present, the main part 

 of the canker knot has seemed to be the production of a single year and, in 

 fact, a one-sided woody excrescence over a place which, even in the pre- 

 \ious year, had formed a wedge-shaped zone of porous, parenchymatous 

 wood tissue, its pointed end toward the interior. In so far two years are 

 necessary for the comi)letion of the canker knot. If the above mentioned, 

 wedge-shaped zone is traced backward to the annual ring of the previous 

 year, it will be seen that this originates in a brown, slender place in the first 

 spring wood. 



The adj(jining anatomical picture, b'ig. 141 C, will facilitate the expla- 

 nation. The whole figure C is a radial section of the second annual ring 

 from a Spirea stem and contains the tissue zone which is preparing to 

 develop into the real canker swelling. The line / to ff represents the strip 

 of changed tissue, which in its further development in the following year, 

 will have become a complete canker knot. The tissue shown at a is the 

 autumn wood of the first annual ring. Xo disturbance has been observed 

 in the wood body of this first annual ring, just, as in the canker of the grape, 

 the first annual ring has a perfectly normal structure. The wood of the 

 second annual ring (/') at first began a normal develo])ment and continued 

 it ui) to //. 



At this time occurred some disturbance wliich produced the cleft {d), 

 and browned its edges {c'). The time this split was produced must have 

 lieen that of the greatest formation of new wood for, only a few cell rows 

 farther, we find that the split is closed at h, and the annual ring has grown 

 further with the formation of groups of normal parenchymatous elements 

 (p). Only a single cell-row (k) forms a radial stripe, with shorter cells 

 containing wider lumina. Now the abnormal wood stripe, instead of dis- 

 appearing as the annual ring matures and increases in width, grows broader, 

 since more and more cells take part in the changed form of construction 

 (kh). Thus the disturbance advances until the second annual ring is lin- 

 ished and then begins, to a renewed extent, in the spring zone of the third 

 annual ring {c-c). 



V.ven when the second annual ring is finished, the stripes of the begin- 

 nings of the canker may be seen to project as slight elevations above the 

 periphery of the remaining wood ring. In the spring of the third year the 

 new formation at this place is so luxuriant that the rapidly growing canker 

 knot, strengthened by the equally rapidly excrescent part of the bark (k I), 

 ruptures the normal liark (r) at sp and now grows further, as it were, as 

 a foreign structure, in order, after some weeks, to end its growth, being a 

 complete canker knot 1 to 2 cm. thick. 



