16 MICROSCOPIC PLANTS. (CHAP. 
ing branches are given off, and we soon have a com- 
plete colony of moulds, 
Another mould (Mucor mucedo) hears a consider- 
able resemblance to the last, though the plant is not 
divided into a number of cells, 
but is one continuous cell. A 
large round cell is formed at the 
summit of an aerial hypha, the 
protoplasm of which divides into 
a number of smaller cells. These 
are the spores, and the large cell 
in which they are formed is called 
the sforangia. The wall of the 
sporangia bursts and scatters the 
spores. 
An interesting thing about 
many of these fungi is what is 
termed polymorphism, or many 
forms. Thus Torula is believed 
by some scientific men to be merely a form of some 
mould like Penicillium or Mucor, and that the differ- 
ence in form depends upon the substance they are 
growing on or in. Penicillium has been found to 
give rise, under certain conditions, to another form 
previously ranked as an independent species of Euro- 
tium. In Puccinia graminis (fig. 18) and Atcidium 
berberidis (fig. 19) there is a remarkable inter- 
change of form, according to the plant on which it 
is parasitical. Puccinia is the “rust” which farmers 
find so destructive to wheat, whilst “czdium is a 
parasite upon the Berberry. There is an old agri- 
cultural belief that Berberry bushes near cornfields 
