112 



PLANT MORPHOLOGY 



slender branch that is cut off by a basal wall. It gives rise to a number 

 of uniciliate sperms that escape and swim in the water. A sperm enters 

 the oiigoniiiin through a terminal pore and unites with the egg. The 

 ]\Ion()l)lophari(hiles are remarkal)le in being the only fungi with swimming 

 sperms. According to the species, the zygote may mature either inside 

 or outside the oiigonium. It becomes a thick-walled resting cell, later 

 producing a new mycelium. 



3. Plasmodiophorales 



This order comprises 8 genera and 23 species, of which the best known 

 is Plasmodiophora brassicae, a parasite attacking cabbages and other 

 crucifers and causing a disease known as clubroot. Another member of 

 the group, Spongospora subterranea, is responsible for a disease of potatoes 



A B 



Fig. 88. Section of a portion of a cabbage root, showing two stages in the development of 

 Plasmodiophora brassicae within the cells, X250. A, plasmodium completely filling a cell; 

 B, spore formation. 



called powdery scab. The Plasmodiophorales, once regarded as para- 

 sitic myxomycetes, are now generally considered to belong to the lower 

 phj^comycetes. 



When cabbages are attacked by Plasmodiophora, the root undergoes 

 a marked enlargement. The cells of the root are invaded by biciliate 

 zoospores. The cilia, attached anteriorly, are of unequal length.^ The 

 zoospores lose their cilia and become amoeboid, migrating directly 

 through the cell walls of the host. An amoeboid cell (myxamoeba) gives 

 rise to a multinucleate plasmodium (Fig. 88A). This soon undergoes 

 cleavage into many uninucleate, walled cells, each of which is said to 

 form four or eight biciliate isogametes that pair and fuse. The amoeboid 

 zygote enlarges, becomes multinucleate, and migrates into another cell 

 of the root, which it finally fills. The diploid nuclei of the young plas- 

 modium continue to divide until the two reduction divisions have 

 occurred. Then a number of small spores are formed, each with a cell 



1 Until recently it was thought that the zoospores were uniciliate and, chiefly on 

 this basis, Plasmodiophora and related forms were classified as a family under the 

 Chytridiales. 



