THE SYNCHYTRIAN NUCLEUS 87 



intermittently to form chromidia in the reticulum, whereas 

 in this thistle- tSynchytrium the nucleolar substance has the 

 appearance of being converted continuously into a chromatic 

 reticulum in which separate chromatin granules are not 

 recognisable. 



The next stage of the direct sporangium is that shown 

 in Fig. 23, /. The nucleoli are not yet wholly absorbed, 

 but the reticulum has changed, having become finely granular 

 and less deeply stained, and in every part definite equal 

 grains of chromatin are evenly distributed in the reticulum 

 of the developing sporangium. Stages between that just 

 described and the formation of separate zoospores, Fig. 23, 

 h, i, are not to be traced clearly in the sections of the thistle. 



The zoospores are typical ; but in this case the wall of 

 the direct sporangium is much thinner than that of the 

 soral sporangia. Beside the intracellular parasites in the 

 thistle are larger intercellular bodies, stained purple and of 

 a texture resembling akaryote plasmodia. These structures 

 call for further examination. 



The Nuclear Cavity. With regard to the nuclear cavities 

 Fig. 23, d, and /, they are obviously nothing more than 

 sap-vacuoles ; and in this relation it is of interest to recall 

 the view expressed in 1903 by A. A. Lawson, and supported 

 by careful cytological observations. Lawson concluded that 

 the nuclear cavity of the cells of the higher plants is only 

 a water- cavity similar to a cell-vacuole ; in other words 

 the nuclear membrane is a cytoplasmic structure. 



