64 + 



BULI.ETIN 8l. 



ly magnified, as found in the crotch of two small limbs. It will be 

 noticed how deep is the crevice which extends downward from 

 the point at which the two parts are apparently united. Another 

 interesting feature of the diagram is the variable thickness of the 

 layer. Where fully exposed to the air it is comparative! 5^ thick, 

 but within the crevice it is much thinner, especially so in a few 

 places. Diagram C shows the layer magnified still more, a sec- 

 tion having been taken at the point c in B. The center cells are 

 flattened to such an extent that they cannot be plainly seen. 

 Figure 2 represents a highly magnified termination of another 



^crotch taken from a Mo- 

 rello cherry tree the wood 

 being two and three years 

 old. The point b in this 

 diagram corresponds 

 with b in diagram B of 

 Fig. I ; it is the lower 

 extremity of the cork 

 layer. But this fork shows 

 a fissure which prolongs 

 the crevice naturally 

 made. The layer of cork 

 ce Is has been separated, 

 and an opening made in- 

 to the cortex lying un- 

 derneath. The cells 

 surrounding the fissure 

 were tested for corky tissue and also for lignin, or woody fibre. 

 These two substances were found not to be present. The threads 

 of mycelium would therefore have little trouble in gaining an 

 entrance. The fissure seen below b was not a new one, for the cell 

 walls surrounding it were darkened as by exposure to air, and it 

 is probable that they exist quite commonly in the forks of Morello 

 cherry twigs. 



The manner in which the black-knot fungus gains an entrance 

 into the forks of the smaller branches may thus be made fairly clear. 

 A spore lodges in the crevice between the two branches ; the 

 action of the wind and the weight of the branches, especially 

 when wet and bearing foliage, open the crevice more or less and 



2. — Lodgement for spores in a crotch. 



