554 



THE PATHOGENIC 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 unafiected. A 



Fig. 330. — PkyllosipJum arisari on Arixarum vulgare. A, Leaf with yellowish 

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

 filament; V, spores. (After Just and Engler-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 

 Just only from December to April, then they disappeared, 

 indicating that the algal spores must have a resting-period 

 outside of the Ai'isarum, and return to young plants again in 

 autumn. 



Phjrtaphysa Treubii Weber et v. Bosse. Forms characteristic 

 galls on a species of Pilea in Java. 



2. PHAEOPHYCEAE. 



These algae live only in other living algae, and are not en- 

 dophytic in higher plants. 



