344 Brown.—Studies in the Physiology of Parasitism . I, 
The acidity is only given in a few cases; in the majority of plant 
juices it cannot be measured by a process of neutralizing on account of the 
formation of precipitates, and the presence of colour, opalescence, &c. 
The above figures can only be interpreted in a very general way. 
When the plant juice forms a clear solution we must attribute its action on 
the fungal extract to the chemical substances contained in it. The high 
degree of retardation produced by the higher concentrations of lemon juice 
is obviously referable to its high acidity (cf. action of citric acid); in the 
others we must look upon the effect as being due to the combined action of 
acids, salts (probably including esters), &c. Where the plant juices are 
turbid, it is only possible with the data yet to hand to speculate as to 
the interpretation. Here the fine debris present may disturb the equilibrium 
of the colloidal enzyme in the solution, and may therefore be expected to 
increase the retarding effect of the plant juice. 
The results with bean juice were very surprising in view of the great 
susceptibility of that plant, so it was necessary to examine this case in more 
detail. The expressed sap of bean leaf forms a deep green liquid, highly 
turbid. A portion of this turbidity can be removed by high-speed centri¬ 
fuging, as also by passing through filter-paper. After the centrifuged liquid has 
stood for a short time (an hour) a further sediment can again be obtained by 
centrifuging, and so on. The crude juice can be driven through a Chamber- 
land filter, and comes through free of turbidity. Soon, however, it is found 
that centrifuging causes a sediment in the filtered liquid. This process of 
precipitate formation is no doubt partly one of agglutination ; it is also due 
in part to the oxidase action which takes place at the free surface, and which 
results in the formation of a black precipitate. In the crude juice, therefore, 
there is present a copious turbidity which cannot be got rid of by simple 
filtering. When the juice is boiled a very large precipitate is thrown down, 
and the juice now remains permanently clear. This boiled juice has a much 
smaller retarding effect than the crude juice. 
Apart from the retarding- action of substances contained in the bean 
extract upon the fungal extract, the former induces a marked change upon 
the potato discs themselves. This phenomenon has already been alluded 
to (p. 326, foot-note), but the present constitutes a much more striking 
manifestation. Discs of potato which have been kept for some time in bean 
juice possess diminished sensitiveness to the action of the fungal extract. 
Thus in one case a potato disc which had been kept for twenty-four 
hours in bean extract was found to be apparently unaffected by freshly 
prepared Botrytis extract after a two days’ action, while a similar disc 
which had been kept for twenty-four hours in water was completely 
disintegrated by the same extract in sixty minutes. This ‘ hardening ’ 
effect is much greater in the case of the expressed sap of the leaves than in 
that of the stem; it is also more marked with potato than with turnip 
