86 PKOTISTS AND DISEASE 



also do the special capsules of the soral sporangia. Of the 

 latter some are somewhat shrunken by dehydration as 

 shown in Fig. 23, a. 



In some sections clusters of three sori are found. It 

 will be noted that the cells around the sorus do not project 

 to form a rosette. The rigid nature of the host tissue 

 caused the parasitic elements to fall out of a number of the 

 sections so that a continuous series of sections of any one 

 element was not obtainable, still it is easy to recognise two 

 separate processes ; one the formation of sori, the other 

 the formation of single or direct sporangia in some of which 

 the zoospores are completely formed. In this thistle the 

 soral sporangia and the non-soral (here termed " direct ") 

 might belong to different species, though this would not 

 rob their minute structure of interest and meaning. The 

 earliest stages are not present : the youngest direct 

 sporangium met with is shown in Fig. 23, c, d, where three 

 nuclei are seen, all having relatively large nucleoli. The 

 nuclear membranes are merely surfaces of plasm bounding 

 sap-vacuoles in each of which a nucleolus lies eccentrically. 

 The nucleoli have the same general characters as those of 

 S. endobioticum. What looks like a vacuole in Fig. 23, e, 

 is really a blunt process like others which are seen in profile 

 in the same element, studded at their surface with granules, 

 the latter on the left side being continuous with the adjoining 

 part of the nuclear membrane. In 8. endobioticum the 

 nucleolus (according to Curtis) discharges chromatic matter 



