223 



in an immature condition which is especially susceptible to the disease, while 

 seedlings forced by heat pass rapidly through the susceptible stage and thus 

 escape the danger." 



In this explanation, after many modifications of Frank's original state- 

 ment, is expressed the theory that besides this specific excitor of disease, 

 Phoma, a definite degree of susceptibiUty of the beet seedling must exist for 

 the production of rootblight. Sorauer held this point of view earlier since 

 he proved that rootblight can exist without the presence of Phoma and that, 

 instead of this, bacterial growth accompanies the disease. We owe the most 

 thorough investigations of the bacteria of rootblight to Hiltner, whose recent 

 studies we will consider with great thoroughness after sketching Stoklasa's 

 theory. According to Stiffs statements (loc. cit. p. 17) Stoklasa admits that 

 bacteria can produce rootblight in beets, and he considers the following 

 species capable of doing so: — Bacillus subtilis, B. liqiiefaciens, B. fluore- 

 scens liquefaciens, B. mesentericiis z'ulgatus and B. mycoides; Linhardt de- 

 clares the latter to be the essential cause of injury. Recently Pseudomonas 

 campestris has been added to these. Stoklasa considers that the above men- 

 tioned atmospheric and soil conditions produce a predisposition in the beet 

 seedlings. He turned his attention especially to oxalic acid, normally formed 

 by the life process of the plant as potassium oxalate. Soluble oxalates, 

 which act as poisons, are transformed into an insoluble calcium oxalate, if 

 calcium oxide can be taken from the soil by the root hairs. By thus neut- 

 ralizing the oxalic acid its retarding action on the process of assimilation 

 ceases and the plant recovers. If much nitric acid is present in the soil or is 

 added in excess (strong fertilisation with nitrate of soda), an hastened de- 

 velopment takes place at any rate, but at the same time the oxalic acid con- 

 tent increases. In such a case, if the young beet plant cannot take up suf- 

 ficient calcium, it becomes predisposed to rootblight. 



As already said, we owe the most thorough study of the relation of bac- 

 teria to this disease to Hiltner and Peters^ These investigators made a 

 number of experiments and found that there are soils which almost never 

 show any rootblight and, conversely, there are others in which the disease al- 

 most always appears. They concluded from this, that many soils are in a con- 

 dition to lend a certain protective power and they perceive that this pro- 

 tective peculiarity is the ability of the immunizing soil to provide the outer- 

 most cell layers of the roots of the beet seedling with such micro-organisms 

 as can prevent the penetration of fungi and bacteria producing rootblight. 

 Hiltner and Peters call this protective sheath, which they had observed 

 similarly in peas, "Bacteriorhiza." If its formation be prevented by steri- 

 hzing the soil and killing the protective soil organisms, in case the seed had 

 not been previously sterilized, the fungi and bacteria causing rootblight 

 could enter the young seedlings and destroy them. 



1 Hiltner, L., and Peters, L., Untersuchung-en iiber die Keimlingskranklieiten 

 der Zucker- und Runkelruben. Arb. d. Biolog. Abt. f. Land-und Forstwirtsch. am 

 Kais. Gesundheitsamt, Vol. IV, Part 3, 1P04, p. 207. 



