Tube-cultures.
In order to obtain more decisive answers to such questions as Are any of the results obtained on plants in the open, or merely covered with bell-jars and so forth, due to spores accidentally introduced, or to mycelium, &c., already in the plant ? a number of infections were made on seedlings germinated and grown anti-septically in tubes as follows :
As a preliminary trial, to test whether grass seedlings would live long enough in tubes and show the results of infection, I placed seed- lings, all similar, and bearing three to four leaves, in test-tubes, the roots, carefully washed free of soil, resting on wet cotton wool, and the orifice of the tube plugged. The results of two series apparently showed that spores from B. mollis infect B. mollis and B. sterilis[1] B. secalinus and B. arvensis, but not B. inermis, while spores grown on B. sterilis only infected B. secalinus and B. arvensis (see Table I).
It is a striking fact that under the conditions afforded by these closed tubes, with their stagnant moist atmosphere, spores from B. sterilis apparently succeeded in infecting B. secalinus and B. arvensis. The pustules were small, and it is significant that it took 14 days' incubation to establish the mycelium, against 9 days in the case of spores from B. mollis.
There are, however, other slight departures from the usual course of events e.g., spores from B. mollis failed on B. arvensis and succeeded on B. sterilis which may point to the probability that these uprooted seedlings in tubes are not in their normal conditions. The facts seem worth recording in this connection.
I varied these tube-cultures by placing them, after infection, under cuvettes filled with copper sulphate (blue light) or bichromate of potassium (orange light). The results were somewhat contradictory as regards the blue, but it did appear that the spores coming from B. mollis can infect B. sterilis as well as B. secalinus, as the following Table II shows.
It is important here to bear in mind the origin of the seedlings themselves, however, as a serious source of error was discovered in these uprooted seedlings, previously exposed to possible infection in the open.
The same criticisms, in fact, apply to these transplanted garden seedlings in tubes as I have given elsewhere regarding such experiments in pots, viz., it was always possible that the seedling was already infected, or had wind-blown spores on it when removed from the beds. At the same time the long incubation period, 14 days in some cases, fails to support this.
- ↑ An exceptional circumstance, as numerous experiments have shown, and itself sufficient to raise the criticism which follows.
