THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 197 



In other genera the spores derived from a plasmodium do not separate, 

 but form a group with a more or less regular and characteristic shape, and 

 it is chiefly by this means that the genera are distinguished from one another. 

 In Spongospora suhterranea the spore mass forms a ball of irregular shape 

 which has hollows in it and can be compared 

 to a sponge, whence the name Spongospora 

 was derived (Fig. 188). 



The spores are liberated by the break- 

 down of the host tissue and are thus set free 

 into the soil. Here they germinate to pro- 

 duce zoospores, which are pyriform in shape 

 and possess each two flagella, one directed 

 forwards and another, very much shorter, 

 which may be directed sideways or back- 

 wards. These zoospores have the power 

 of penetrating the root hairs, and once Gamet- 

 inside give rise to gametangia (Fig. 189), °"?'""' 

 which are very small spherical bodies, the 

 contents of which divide into a small number 

 of motile isogametes. These fuse in pairs 

 to form fresh myxamoebae. Whether the 



migration from the root hairs to the root 



, , , p r r • r ^u FiG. 189. — Plasmodtophora brasstcae. 



takes place before or after fusion ot the Gametangia in root hair of host 



gametes is not known, but the myxamoebae plant. 



are found mainly in the meristematic cells of 



the young root, where they stimulate active cell division and in this way are 



themselves widely distributed in the host tissue. 



In Spongospora suhterranea it is the cortical tissue of the tubers which is 

 most characteristically attacked and the hypertrophy is restricted to the 

 formation of scabs, the surface of which becomes powdery as the spore- 

 masses mature. When the roots are attacked, however, hypertrophy may 

 sometimes be very marked, and the effect of the disease resembles super- 

 ficially the Wart Disease, Synchytrium endobioticiim (see p. 203). (Fig. 190.) 



The cytology of Plasmodiophora (Fig. 191) and indeed all the members 

 of the order is peculiar and unique. During the development of the 

 Plasmodium all the nuclei within it divide simultaneously by a process termed 

 protomitosis, which differs from a normal mitosis in certain features. 

 The metabolic nucleus does not possess a typical reticulum but instead 

 the chromatin is disposed in a peripheral zone, leaving a clear, central, 

 nuclear vacuole in which lies a single spherical karyosome. During the 

 prophase of the division the peripheral chromatin becomes denser and 

 the nuclear membrane becomes drawn out into an ellipsoidal shape. At 

 metaphase the chromatin forms a median ring surrounding and enclosing 

 the karyosome, which now becomes drawn out and finally dumb-bell shaped, 

 the nuclear membrane persisting throughout the division. The chromatin 

 ring now splits into two rings which move away from one another, keeping 



