108 PLANT MORPHOLOGY 



Labyrinthuleae. In this little-known group the vegetative cell is 

 spindle-shaped with tufts of pseudopodia at the ends. When the cells 

 come in contact, their pseudopodia generally fuse, the union of numerous 

 cells producing a net-like structure called a net-plasmodium. The 

 individual cells, retaining their identity, appear to glide along the threads 

 of the net in limited movements. During this stage they feed, increase 

 in size, and undergo repeated division. In dividing, the cells become 

 constricted at the middle and then separate, but are held together by a 

 protoplasmic strand. At the close of the vegetative stage, the cells 

 collect into sessile or stalked masses and become encysted. In some 

 species the spores have cell walls, in others not. Later the spores germi- 

 nate, freeing one to four spindle-shaped cells with polar pseudopodia. 



3. PHYCOMYCETES 



The Phycomycetes, or alga-like fungi, comprise the first group of "true 

 fungi" (Eumycetes), as the higher fungi are often called in contrast to 

 the bacteria and myxomycetes. All the true fungi have a definite nucleus 

 and nearly all have a characteristic plant body called a mycelium. This 

 is composed of branching filaments, each branch being a hypha. The 

 hyphae may be either loosely or compactly interwoven. With few excep- 

 tions, the Phycomycetes are characterized by an absence of cross walls 

 in the mycelium, and so, as in Vaucheria, the plant body is a coenocyte. 

 Their spores are borne in indefinite numbers within a sporangium. The 

 origin of the Phycomycetes is not clear. They may have evolved either 

 from colorless flagellates or, through loss of chlorophyll, from the Chloro- 

 phyceae, a group which they resemble in both vegetative and reproduc- 

 tive features. A number of Phycomycetes cause diseases of economic 

 plants, such as cranberry gall, brown rot of lemon, downy mildew of 

 grape, and late blight of potato. The group is a relatively small one, 

 numbering about 1,000 species. These are included in seven main 

 orders: Chytridiales, Monoblepharidales, Plasmodiophorales, Sapro- 

 legniales, Peronosporales, Mucorales, and Entomophthorales. 



1. Chytridiales 



The Chytridiales are the simplest of the Phycomycetes. Nearly all 

 of them are parasitic, many living on fresh-water algae and others 

 attacking seed plants growing in moist situations. The order includes 

 about 65 genera and 300 species, the best-known forms being Chytridium, 

 Olpidium, and Synchytrium. 



Chytridium. A common species of Chytridium attacks the green alga, 

 Oedogonium. A uniciliate zoospore comes in contact with an oogonium 

 of the host, loses its cihum, and sends into the host cell a tube through 

 which food is absorbed. This tube represents a weakly developed myce- 



