Colorado Plant Diseasb:s 
11 
111 order to thorougldy understand the nature of a plant disease 
it is necessary to know something of the nature of the organism 
that produces it. It is, therefore, important that we learn some- 
thing of the nature of fungi before we take up the study of the 
diseases which they produce. 
Fungi, for convenience of study, are divided into four large 
groups, the members of each group having a number of important 
characteristics in common. These groups are known by the follow- 
ing names: 1. Phycomycetes, 2. Ascomycetes, 3. Basidiomycetes, 
4. Fungi Imperfecti. 
The Phycomycetes include 
the simplest and most poorly 
developed forms of fungi. In 
some species hardly any my- 
celium is developed and the 
plant is extremely simple in 
structure. The mycelium never 
has any cross walls but consists 
of a continuous tube filled with 
a jelly-like substance (proto- 
plasm). The mycelium of all other groups is divided into short cells 
by means of cross walls. 
Nearly all the members of this group are water loving and grow 
best in very moist situations. For this reason a peculiar kind of 
spore which has the power of swimming about in water is commonly 
Spores of a Phycomycete. a — A mature 
sporangium. b — The same sporangium 
breaking open to free the motile Zoo- 
spores. c — Zoospores. (After de Bary. ) 
