LIFE-HISTORY OF PENICILLIUM. 347 
when once fully formed never occurring. The end-cell is 
capable of forming lateral branches at different parts, 
and these appear so near to the apex that for a short 
time they appear as if formed by dichotomy—which is 
never the case—instead of being monopodial. While the 
end-cell grows and forms branches, the older hyphal cells, 
which have ceased to grow in length, form new lateral 
branches. These have a definite position, and are generally 
formed immediately below a transverse septum. On the 
branched hyphe, which constitute the mycelium, there 
are two kinds of lateral branches—the one formed by 
the branching of the end-cell, and inserted about its 
middle, while the others are lateral branches of the hyphal 
cells, and are inserted close to the septa. It is not neces- 
sary, however, for the hyphze to branch, and in many 
cases both kinds of branching may be arrested and long 
single threads be produced in consequence. ‘The lateral 
branches have the same monopodial mode of development as 
the main axis. The whole system of branches of the myce- 
lium develops centrifugally from the spore as a centre. At 
the first glance we observe that throughout the mycelium 
the hyphe are of the same thickness, and when first formed 
the side branches are of the same thickness as the main 
axis, the youngest side branches being, when first formed, 
of the width of the main branch. The diameter of the hyphe 
varies in different cases, being about 0:0071 mm., and when 
poorly fed the diameter may only be about 0:0040 mm. 
The contents of the mycelium resemble those of the ger- 
minated spore. They consist of an exceedingly finely 
granular protoplasm with very small vacuoles. When 
magnified less than 3800 diameters the hyphe are 
strongly refracting, but cease to be so when dead. Violent 
shaking causes changes in the large rank mycelium, the 
hyphe rupturing near the apex by endosmotic action. ‘The 
protoplasm escapes at these places, changes its appearance, 
and becomes dark and granular, but does not mix with the 
nutrient solution in which the mycelium lies, like the proto- 
plasm of the higher plants. A wall soon forms round the 
protoplasm, and its further escape is prevented. This growth 
is somewhat like the formation of cork in wounds of higher 
plants. Brefeld has also observed a similar process in the 
mycelium of Mucor and Entomophthora musce. 
It is a special peculiarity of the mycelium of Penicillium 
that at certain places the hyphe fuse together and anasto- 
mose. ‘The fusion is only seen properly in small, slightly 
branched mycelium, and when the hyphe are isolated and 
