W. ZOPF ZUR KENNTNISS DER PHYCOMYCETEN. 119 



this method of swarm-spore development. The free condition of 

 the zoospores is strikingly short. As soon as they escape they 

 make for the nearest Synedra and fasten upon it ; too many as a 

 rule, often 40 or more. The great increase of the fungus is suffi- 

 ciently explained by their number and rapid development into 

 sporangia, which is completed in from 24 to 48 hours. 



The effect of the fungus on the host-plant is first manifested by 

 an alteration in the shape and position of the chlorophyll bands. 

 They recede from the walls, contract in direction of their length, 

 and become closely applied to the parasites ; in some instances 

 they form curved strands or lumps. Simultaneously the nucleus 

 is dissolved and the plasma contracted. Later on, in consequence 

 of the pressure exercised by the parasites, the valves fall asunder. 



As regards the systematic position of Ectrogella, it may be 

 determined with certainty by the sporangial fructification. The 

 form of the mycelial sacs undoubtedly coincides with that of the 

 Ancylistece ; nevertheless, as the entire vegetative partis transformed 

 into a single sporangium, and the swarm-spores are spherical and 

 uni-ciliate, it has a distinct affinity to the Olpidiece. It stands to 

 Ancylistes, as Olpidiopsis to Myzocytium. 



EXPLANATION OF PLATE. 



All figures magnified 300 diameters. 



Fig. 1. Synedra, with a large and almost fully developed mycelial sac. 



2. Synedra, with two young mycelial sacs. 



3. Synedra, with numerous nearly mature small parasites, of 

 various size and shape. 



4. Stouter mycelial sac, constricted at its upper third and tapered 

 at both poles. It contains two vacuoles. At O is seen the 

 papilliform rudiments of two excretory pores. The valves of 

 the Synedra are slightly driven outwards by the expansion of 

 the parasite. 



5. Synedra, with one valve separated. The five sporangia are free, 

 and exhibit various stages of development. At A, the spores 

 are already mature. 



6. Valve view of a Synedra. One (A) of the sporangia shows the 

 excretory papillae pushed past the separated girdle-band. 

 A few Ectrogella spores have just fastened on the outer 

 membrane. A, a single swarm-spore. 



7. Girdle-band view of a Pinnularia, with a nearly mature parasite. 



8. Empty sporangium, with the excretory pores arranged in two rows. 



9. Empty sporangia, the upper having two, the lower only one 

 excretory pore. 



»» 



»> 



»> 



J5 



