406 PLANT GROWTH SUBSTANCES 



in the intercellular spaces between cells that normally never would divide 

 again. From histological studies, Riker (14) showed in 1923 that within 

 two days the cell walls increased in thickness near the bacteria, the 

 entire cells became larger, the nuclei apparently took up a position 

 near the bacteria. New cell walls were laid down within four days in a 

 plane perpendicular to the location of the bacteria. This time interval 

 parallels that determined by Braun (2) with heat treatments. Further 

 cell divisions followed rapidly and in a disorganized manner until a gall 

 developed. 



Activity of bacteria in culture. — What happened above perhaps may 

 be clarified by examining what the bacteria have done in culture media. 

 There the details have been followed with considerable precision. At the 

 same time we recognize the difference between a culture tube of glass 

 and a capillary space in the plant. Among the early changes the bacteria 

 induced in culture, after the lag period, was the lowering of the oxidation- 

 reduction potential of the medium (21). Hydrogen-ion concentration 

 of the cultures was changed little, if any, except in certain cases, when, 

 for example, complex nitrogen compounds were used also as carbon 

 sources (24). Osmotic pressures became less (19) as sugar was used 

 and viscosity was lowered (i). 



Among the chemical products formed in culture in addition to carbon 

 dioxide, the most abundant was a polysaccharide. This has been charac- 

 terized through a series of investigations and shown to be toxic under 

 certain conditions (7). Substances related in size have proved toxic in 

 direct relation to their molecular size (8). In passing perhaps we should 

 mention that toxic substances at sublethal concentrations frequently 

 may be stimulating. Small amounts of various other substances have 

 been isolated, including phosphatides (4), thiamin, riboflavin, biotin, 

 pantothenic acid (13), and auxin. The last has received much special 

 attention. 



Groivth substances in cultures and galls. — The production of growth 

 substances in cultures of virulent and attenuated bacteria has been 

 approximately the same when peptone containing tryptophane was a 

 part of the medium (10). However, in some recent unpublished work 

 with Hodgson, Tsui, and Skoog, it appears that in a synthetic medium 

 the virulent culture produces more growth substances than the attenu- 

 ated culture. This is noteworthy when correlated with the different 

 symptoms induced by these cultures on decapitated tomato plants (9). 



