42 A TEXT-BOOK OF BOTANY. 



a definite shape and size, which is called an ASCUS, and which 

 contains a definite number of spores, which is two or some multi- 

 ple thereof. 



The Basidiomycetes are the most highly developed Fungi, 

 producing large fruit bodies, such as are seen in mushrooms, toad- 

 stools, and puffballs. They are characterized by producing spores 

 (basidiospores) on special hyphae. The spores are usually four 

 in number, and the spore-producing organ is known as a BASIDIUM. 



The Fungi Imperfecti constitute a group of Fungi which, 

 while having certain natural relationships with the other types 

 already considered, yet do this so imperfectly that they are brought 

 in a class by themselves. The complete life-cycle is not in all cases 

 known, and future studies will probably distribute them among 

 the other principal groups. 



PHYCOMYCETES : ALGA-FUNGI The plant body of 

 the Phycomycetes consists of a mycelium which is unsegmented, 

 more or less thread-like and sometimes considerably branched. 

 Reproduction takes place by means of several kinds of spores, and 

 by reason of the production of two kinds of sexual spores they are 

 subdivided into two important groups. These are ( I ) the Oomy- 

 cetes, which produce oospores, and (2) Zygomycetes, which 

 produce zygospores. 



Saprolegnia. Probably one of the best representatives of 

 the Oomycetes is the group of water molds known as Saproleg- 

 nia, which are aquatic in their habits and are both parasitic and 

 saprophytic, occurring on living fish, insects, crayfish and decay- 

 ing plants and animals as well. The plant body consists of a 

 mycelium which may be simple or branched, sometimes forming 

 a dense mass (Fig. 21, A). Like the alga Vaucheria, it produces 

 both swarm spores (zoospores) and oospores. The swarm spores 

 (Fig. 21, B, C) are produced in sporangia formed by the pro- 

 duction of a partition wall at the end of a hypha. The sporangia 

 are either cylindrical or spherical, and contain numerous zoospores 

 which have two cilia at one end. These spores are peculiar in 

 that after their escape from the sporangium they swim about, 

 then come to rest and take on a wall, after which resting period 

 they develop two cilia on the side, again move about, and germi- 

 nate when they find a suitable host. 



