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E. A. ANDREWS. 



black aggregates crowded on the young leaves nearly to the tip 

 where only the newest leaves were as yet unoccupied ; by August 

 1 8 the extension of the Folliculina hosts had ceased. The tips 

 of the growing Elodea were now bare or free from Folliculina 

 back some twenty leaves from the tip and many of the old dwel- 

 lings on the lower leaves were deserted. These dense black 

 colonies on old leaves contained in fact but few living Folliculinas. 



FIG. 5. Photograph of natural size sprays of Elodea preserved to show successive 

 phases of colonization in 1914. Spray on left has grown enough to form flower but 

 as yet but a very few .isolated individual Folliculina have settled upon it. The next 

 spray shows scattered tubes all along its length. The third spray shows dense 

 aggregations of colonies even up to the tips of the rapidly unfolding new leaves. 

 The fourth spray illustrates the subsidence in colonization: the new colonies no 

 longer cover the leaves at the tip of the spray but these grow more rapidly than the 

 new colonists occupy them and are left more nearly free from any Folliculinas. 



By August 26 this falling off in the colonization and rapid dis- 

 appearance of Folliculina was most pronounced : the Elodea sprays 

 showed an abrupt transition from the lower leaves black from 

 dense population of tubes, for the most part empty, to the upper 

 leaves only sparsely inhabited with scattered individuals. Evi- 

 dently some sudden change had operated not only to check the 

 previously rapid spread of the Folliculinas onto new leaves but to 



