After a shorter or longer period of development, dependent on the
conditions, branches are sent perpendicularly from the substratum, and
into the air. These branches cease their growth in length, sending
out branches near the tip, which take the same general direction as
the original branch. Each of these subbranches is called a sterigma
(from the Greek word meaning _support_). In vigorous development the
sterigmata may form secondary branches, the whole forming a tassel-like
arrangement. The tip of a sterigma enlarges, a septum forms around the
enlargement, cutting it off from the sterigma, and forming a conidium.
The sterigma develops to the original length and another conidium is
formed, the operation being repeated many times, thus forming a chain
of spores. As the other sterigmata are also forming conidia in the same
manner, a series of these chains is formed close together. After the
cessation of conidial development, the filament below the sterigmata is
disorganized, setting free the conidia. The filament and head together
are called the conidiophore (Greek, dust-bearer).
Penicillium forms spores sexually, but the conditions for their
formation are unknown. Brefeld obtained them by growing the mold on
damp bread placed between two glass plates, and excluding the air.
Lindner obtained carpospores on a wort gelatin culture in a petri dish,
from which the air was excluded. The writer has tried various methods
for obtaining carpospores, but so far without success. Moist chambers
were used with various media, excluding the air. The development of
the mold is seemingly dependent on the amount of air in the chamber
at the time of sealing. After the air is exhausted, the conidiophores
assume fantastic forms, developing only one or a few sterigmata, and
on these one or few conidia. In other cases the conidiophores are
fascicled, in no cases, however, forming the conidia as luxuriantly
as when air is supplied. The hyphæ become clear, much vacuolated,
and develop more septa, and some of the cells become much enlarged.
An enlarged cell will often contain two or three septa, thus forming
cells that are not larger than disks. In cultures from which the air
was excluded from the start, no development took place. In test-tube
cultures sealed with paraffin after twenty-four hours, the mold
developed on the surface of the gelatin, forming a felted white mass,
but no conidia nor carpospores were formed.