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MISSOURI BOTANICAL GARDEN. 



branches grow older and ultimately large swellings are pro- 

 duced. In the older swellings, the small branch stub which 

 gave rise to the swelling has by this time usually completely 

 rotted away and the saprophytic fungi which brought about 

 the decay of the small branch stub have usually by this time 

 grown with considerable rapidity in the dead tissue on the 

 surrounding base of the branch stub, so that in the course of 

 time, deep holes filled with decayed wood matter result, (fig- 

 ure 5, plate 6, and figure 2, plate 5). It not infrequently 

 happens that the healing callus is killed either by fungi, 

 which have lodged in the developed canker, or by frost, and 

 in such cases, a scries of callous lips will appear in these older 

 cankers, (figures 1 and 3, plate 5). Primarily, however, but 

 one callous layer is found, and when this finally succeeds in 

 healing over the wound, the cankers appear on the branch as 

 small, round knobs, completely covered by normal bark. The 

 text figure shows a longitudinal section of a canker in its later 

 stages. From this, the relationship which the dead branch 

 stub bears to the lips of the swelling will be perfectly evi- 

 dent. 



The formation of these branch cankers is presented as an 

 interesting instance of the manner in which pathological con- 

 ditions may arise in forest trees without the direct interfer- 

 ence of other living organisms, either fungi or insects. 



EXPLANATION OF PLATES. 

 Plate 5. — Cankers on rhododendron branches. 



Plate 6. — Showing development of cankers around the base of a dead 

 branch. Figures 1-5 show successive stages from the recently dead 

 branch to the fully formed cankers. 



