554 



THE rATHOGENIC ALGAE. 



Parts of the leaf and petiole inhabited by this alga appear 

 externally as yellow spots. Only one individual alga inhabits 

 each spot, sending its numerous branches into the intercellular 

 spaces. Attacked leaf-cells lose their chlorophyll and starch, 

 the latter being at first replaced by oil. The cells, however, 

 remain alive and turgescent, even when deprived of almost 

 their whole content ; they die, when spores are produced in the 

 filaments. Cells undisturbed by the alga remain unaffected. A 



Fio. 330. — Phyllosiphon arisari on Arisaruni vulgare. A, Leaf with yellowish 

 spots ; B, the branched alga isolated from a spot ; C, spore-formation inside a 

 filament; /;, spores. (After Just and Bngler-Prantl.) 



single leaf may bear a large number of spots, and all the plants 

 of a locality are generally attacked. The spots were found by 

 'lust only from December to April, then they disappeared, 

 indicating that the algal spores must have a resting-period 

 outside of the Arisarvm, and return to young plants again in 

 autumn. 



Phytaphysa Treubii Weber et v. Bosse. Forms characteristic 

 galls on a species of Fi/ca in Java. 



2. PHAEOPHYCEAE. 



These algae live only in other living 

 dophytic in higher plants. 



algae, and are not en- 



