Brown.—Studies in the Physiology of Parasitism. L 331 
In coloured cells, death is as a rule shown by escape of the coloured sub¬ 
stance. This substance may remain as such and impart its colour to 
the fungal extract (e. g. Beet), but more generally it disappears from view 
altogether (e. g. Rose, Viola , and many other petals), though its presence 
may be demonstrated by the addition of a suitable reagent (e. g. an acid in 
the case of the red pigment of Rose petals). In other cases the incidence 
of death is shown by a new development of colour, due to autolysis, i. e. to 
actions taking place which were held in abeyance during life. Such is the 
well-known black coloration produced after death in the leaf of the Broad 
Bean. 
These post-mortem phenomena as witnessed in the various tissues 
experimented upon do not call for detailed description here. It is impor¬ 
tant, however, to state that the post-mortem changes brought about by the 
fungal extract, with one exception which is only apparent, 1 were identical 
with those induced by the action of the fungus itself. Again in all cases, 
where a distinct parasitism of the fungus on a particular host could be 
established, it was found that the tissues of the latter were acted upon 
in a similar way by the fungal extract. These considerations—viz. simi¬ 
larity of ‘ range’, and similarity of effects produced—justify the conclusion 
that the standard extract of the present investigation is a true representation 
in essentials of the active principle of the fungus. 2 It is proposed, therefore, 
on the basis of these results, to put forward the following as a working 
hypothesis— that all the macerating and lethal effects of the fungus can be 
explained on the basis of the properties of the standard fungal extract. 
1 In the case of the leaf of Vida Faba , it was found that the discs, when injected and submerged 
in a quantity of fungal extract, did not show the characteristic blackening produced by the fungus 
itself, but remained for a considerable time quite green, fading later into yellow. There was no 
doubt as to the discs being killed, for within a few hours from the commencement of the experiment 
they became quite limp and rotten, and the cells were no longer capable of plasmolysis. Moreover, 
no blackening effect appeared if these discs were subsequently placed in chloroform vapour. It was 
by reason of this discrepancy that the hypodermic syringe method was first resorted to, when it was 
found that leaves injected with extract according to this method gave the normal blackening. The 
explanation of these appearances is fairly simple. The black pigment is formed as the result of an 
oxidase reaction, in which the source of oxygen is the free oxygen of the atmosphere. The latter is 
available in the case where the leaf is injected by means of the hypodermic syringe ; not so when it 
is injected and immersed in some depth of liquid. In the latter case, at the time and place of 
killing, one of the factors essential for the development of the black colour is to a large extent 
wanting; the two remaining factors concerned (enzyme and oxidizable substance) diffuse out into the 
liquid, where they gradually meet the atmospheric oxygen, so that a development of the black 
pigment takes place slowly in the liquid. This last had, in fact, been noticed long before the 
apparent discrepancy was understood. 
2 In Smith’s experiments, treatment of lettuce leaf with oxalic acid produced a bleaching effect, 
in contrast with the browning effect produced by the fungus and fungal extract. These results have 
been confirmed, and in the opinion of the present writer this disparity in post-mortem effects con¬ 
stitutes a strong objection to the view that oxalic acid is the toxic principle of the fungal extract and 
of the fungus. 
