362 Mr Line, A Note on the Biology of 



of the inner membrane is sometimes extruded, and through the 

 cracks the zoospores, accompanied by and often entangled in, drops 

 of oil, are seen to escape; they were found to remain active for 

 several hours: no fusion of the zoospores was ever observed. The 

 rapid growth of bacteria in almost all the drop cultures greatly 

 hindered the observations on the behaviour of the zoospores. 



Reaction of the host plant to infection. 



The zoospores can apparently only penetrate the host at points 

 where the tissues are relatively unprotected by either cuticle or 

 cork. By far the most common starting points of naturally occur- 

 ring galls are the numerous adventitious buds arising in continuous 

 succession from the woody rootstock of the Lucerne plant. Many 

 of these buds are developed some distance below the ground ; they 

 consist of a small axis and a number of leaf rudiments. The zoo- 

 spores appear to penetrate between the outer scale leaves, and to 

 enter the cells of the young leaves and of the growing point itself. 

 Wherever penetration is effected the host is stimulated to locally 

 increased cell division, the mass of tissue resulting bearing greater 

 or less resemblance to the normal bud, according to the degree of 

 infection. An extensive branching vascular system develops mth 

 the gall in direct communication with the vascular system of the 

 host stem ; the galls are thus hypertrophied buds or parts of buds. ; 

 When first observable they appear as minute, white, shining pro-j 

 jections from the rootstock or from a bud. 



The cell originally entered by the zoospore could often be traced, 

 although it is as a rule rapidly covered in by the division of the 

 surrounding cells. The actual penetration of the host cell by the 

 zoospore has not yet been observed. 



From each point of infection the fungus spreads out radially into 

 the host tissues, invading particularly the thin walled cells which 

 have been developed as a result of the presence of the fungus: their 

 contents are absorbed, and their cavities linked up by the absorp- 

 tion of certain of the walls to form an irregularly branching central 

 cavity inhabited by the fungus. 



Active living hyphae of the fungus are only found in the peri- 

 pheral regions, the older portions of the gall being occupied by 

 developing resting spores and degenerated mycelium. The per- 

 sistent walls of all cells entered become thickened and often 

 curiously pitted; the inner side, in contact ■^'ith the fungus, appear- 

 ing somewhat mucilaginous. 



In sections of growing galls it is noticeable that part of the walls 

 in the path of the hyphae disappear before the hyphae come in 

 contact with them, but rim-hke projections of these walls persist 

 long after the cell is incorporated into the main fungal ca\'ity. 



Growth of the fungus is by no means regular in all directions, 



