LARCH AND SPRUCE FIR CANKER. 29 



and canker holds good in other countries, I am not in a position 

 to state. 



During the early part of May, it is not at all unusual to find 

 the axils of many dwarf branches on the stems of seedling 

 and young larches each occupied by an aphis "foundress" and 

 her stalked eggs. Now, as to whether the foundress actually 

 punc cures the bark at this point or not, I have not been able to 

 determine, but there is always a certain amount of moisture on 

 the bark where a foundress is located, and I have repeatedly 

 produced canker by placing ascospores underneath the foundress. 



Situated on the bark quite near to each foundress is a small 

 patch of white flocculent matter, the centre of which is occupied 

 by a fairly large drop of sap. As evaporation proceeds after two 

 or three days, the resinous portion of the sap alone remains and 

 becomes solid (Fig. 5, PL I.). If spores are placed in this drop 

 when first extruded, and in a liquid condition, canker follows. 



The significance of this white fleck with its central drop of 

 plant sap I have not been able to determine. Its constant 

 presence is alluded to by Buckton (5), who also figures it, 

 without comment or explanation. The one important feature in 

 this connection is to remember that it is a channel through 

 which inoculation can take place. 



Professor Marshall Ward (4) considers that late frosts are to 

 a great extent responsible for the prevalence of canker in larch. 

 I have once observed in the middle of May, a time when seed- 

 lings and young larches are replete with sap, that after a sharp 

 frost, followed by bright sunshine, the branches were studded 

 with minute drops of sap, which eventually hardened into resin. 



Ripe ascospores were not forthcoming at the time when this 

 observation was made, but I can readily conceive that canker 

 woidd have resulted if spores had been deposited on the freshly 

 extruded beads of sap. 



Owing to the comparative paucity of sap in the branches of old 

 trees, a May frost would not be so likely to result in injury. 



In the case of young trees, infection often takes place just 

 above the ground line, and is thus explained by Dr Somerville 

 (8) : — "Not only is infection specially liable to occur low down 

 on a stem on account of the abundance of moisture, but the 

 chances of an outbreak of disease at such a place are also 

 favoured by the frequent presence of wounds near the surface of 

 the ground. These wounds may be caused by lifting the plants 



