226 APPENDIX. 



autumn. They are often observed attached to the walls 

 or window-panes, surrounded by a powdery substance, 

 consisting of the extruded conidia. 



Empusa radicans. The spores form long hyphae, 

 which pierce the transparent skin of the caterpillar of the 

 cabbage-white butterfly. The terminal cells ramify, and 

 fill the body of the caterpillar with a network of mycelial 

 filaments. The caterpillars attacked become restless, then 

 motionless, and death ensues. 



Tarichium megaspermum. The spores are -05 

 mm. in diam., black in colour, and provided with a thickened 

 episporium. They occur at the sides and ends of myce- 

 lial threads, attacking caterpillars (Agrotis segetuin}. 



PHYCOMYCETES. 



Saprolegnia. Colourless threads, forming dense radi- 

 ating tufts, occur on living and dead animal and vegetable 

 matter in fresh water. The filaments penetrate into the 

 substratum, and branch more or less in the surrounding 

 water. The cylindrical ends of threads are shut off by 

 a septum forming zoosporangia, or mother-cells, in the 

 interior of which a number of spherical spores, zoospores, 

 develop. These are set free through an apical opening in 

 the thread, and, after a time coming to rest, give rise to 

 new plants (Plate XXIX., Fig. 4). In the sexual mode 

 of reproduction, a spherical bud, the oogonium, deve- 

 lops at the end of a mycelial thread ; from the thread, 

 small processes or antheridia sprout out laterally towards 

 the oogonium, and blend with its protoplasm (Plate XXIX., 

 Fig. 4). The latter breaks up into a number of oospores, 

 which clothe themselves with a membrane, while still within 

 the mother-cell, and eventually being set free, grow into 

 fresh mycelial filaments. The parasite attacks fish and 

 tritons, and produces a diseased condition of the skin, which 

 may be ultimately fatal. In salmon it produces the com- 

 mon disease of salmon. 



Peronospora infestans. Mycelium, -005 mm. in 





