Bark Canker Disease of Apple Trees. Grace G. Gilchrist. 235 
No hyphae are found in the browned zones at first but as 
one follows them back to the older infected parts the extent of 
the browned zones increases, the cortex dies, wound gum 
appears in the wood, and finally mycelium gradually becomes 
visible as has been described above. 
4. Description of conidia and their measurement. 
The majority of the conidia are oval or slightly allantoid in 
shape, although occasionally they are found to be quite curved, 
with the result that at one time this fungus was thought to be 
Macrophoma curvispora Peck. The end of the conidium nearest 
the conidiophore is somewhat bluntly pointed, while the other 
end is conspicuously rounded. The cell wall is comparatively 
thick. These conidia are unicellular, hyaline, and contain large 
refractive globules varying from I to 3-5 in diameter. The 
latter are stained brown with osmic acid, and distinctly pink 
with Millon’s reagent and are therefore probably proteid in 
character. The conidia vary in length from 25 to 45 and in 
width from g to 18 p. 
5. Description of development of the fructification. 
When the fungus is about to produce fructifications, the 
mycelium at various points just below the epidermis forms small 
stromatic masses. The epidermal layers are thus raised in the © 
shape of a dome, the covering of which still remains unbroken. 
The hyphae in the centre of the stroma now begin to elongate 
perpendicularly to the epidermis and develop into a definite 
column of tissue, which forces the epidermis out. 300-350 p. The 
hyphae round the base of the central column do not increase 
their rate of growth but form a kind of basal disc (Pl. XI, 
figs. 17 a, b, c). The spores are produced in large numbers from 
the hyphae on this disc. A hypha becomes constricted im- 
mediately below its apex, forming a small oval-shaped portion. 
Protein material passes from the hypha into this portion which 
increases in size until it becomes, on an average, 36 by I2p. 
Eventually this portion, which is a conidium, is cut off at the 
point of constriction and the hypha is ready to form more conidia 
in a similar manner (Pl. XI, fig. 17 f). The epidermis remains 
unbroken until the conidia are ripe. The sterile hyphae of the 
central column then grow until the layers of bark break. At 
first only a narrow opening is formed, but as the whole of the 
fructification grows, the slit widens and the bark is pushed 
outwards. The conidia are shed into the space between the 
stroma and the bark. Under suitable conditions of warmth and 
moisture these are pushed out of the opening in such numbers 
