TAXONOMY 
199 
laid down in each ol' these a short distance from its tip and the 
contents of each end cell then becomes a gamete or sexual cell. 
The apical cells of the tips of opposite hyphae then meet, a solution 
of the cell walls at the point of contact takes place and the gametes 
of both end cells fuse to form a zygospore. This enlarges and devel- 
ops a highly resistant wall. After a period of rest, upon coming 
into contact with a nutrient medium, it germinates into an elongated 
sporangiophore which develops a sporangium at its summit. 
Mucor mucedo, another closely allied species, found growing on 
old nuts, fleshy fruits, bread and horse manure, resembles Rhizopus 
nigricans in many respects but differs from it by the formation of 
sporangiophores singly instead of in clusters. 
Thamnidium differs from Rhizopus and Mucor in the development 
of two kinds of sporangia, microsporangia and megasporangia. The 
sporangiophore produces a terminal large megasporangium possess- 
ing a columella and whorls of side branches which bear smaller 
microsporangia in which the columella is frequently wanting. 
Sub-class B. — Oomycetes 
(Sexual apparatus heterogamous) 
Order 1. — Chytridiales. — Example: Synchytrium, a form para- 
sitic on seed plants and forming blister-like swellings. 
Order 2.- — Saprolegniales. — Water molds which attack fishes, 
frogs, water insects, and decaying plants and animals. Example: 
Saprolegnia. 
Order 3. — Peronosporales. — Mildews, destructive parasites, liv- 
ing in the tissues of their hosts and effecting pathologic changes. 
Example: Albugo , the blister blight, a white rust attacking members 
of the Cruciferce and Phytophthora, producing potato rot. 
Class II. — Ascomycetes, the Sac Fungi 
Mycelium composed of septate filaments and life history charac- 
terized by the appearance of a sac called an asciis in which ascos pores 
are formed. The largest class of fungi. 
Order 1.— Protoascales. — Plants with asci borne free or at the 
ends of hyphae, definite fruiting bodies being absent. Each ascus 
