EXCRESCENCES. 223 



Cankers due to insects are found on Apples, 

 the cortex of which is punctured by the woolly- 

 Aphis {Schizoneurd) while the twigs are young, 

 and the wound is kept open by the insects 

 nestling in crevices in the occlusion tissues. 

 Species of Coccus, Lac/mus, and C/icrvies also 

 produce cankers on forest trees. 



Cankers due to fungi usualh" originate in a 

 wound primarily due to an insect puncture or 

 bite, or to frost, the invading fungus hyphae 

 making their way into the wounded tissues and 

 gradually extending more and more into the 

 cambium and the occluding callus. Among the 

 best known of these wound fungi which cause 

 cankers are DasyscypJia WillkoDunii the peziza 

 of Larch disease, Nectria ditissiiiia and A^ 

 cucurbitula on Beech and Conifers ; less common 

 are Scleroderris on Willows, Aglaospora on Oaks 

 and some others. 



Peridennium Pini and Aecidiiivi elatiniDii also 

 cause cankers under certain conditions, as also 

 does Gymnosporangium, but in these cases the 

 fungi are more truly parasitic. 



In some cases e.g. xA.sh, Pine, Olives bacteria 

 are concerned as associated organisms in the 

 cankering of trees. 



Burrs or Knauers are irregular excrescences, 

 principally woody, with gnarled and warted sur- 

 faces. They are frequently due to some previous 

 injury, such as the crushing or grazing of cortical 

 tissues by cart-wheels. The excitation of the 

 tissues thus wounded results in the development 



