who considers the attachment of these fatal fungi as a secondary cause of mildew, and as arising from a previously diseased state of the plant, which he seems to think, as I do, would resist their insidious advances in a healthy and sound state. To the question, "what occasions these diseased secretions? to what cause may we attribute the previous injury of those vessels by internal disease?" To these questions he answers, "I believe we must be assisted by the analogy of vegetable with animal life. What more likely to be a leading cause than the stimulus of heat returning perhaps in a greater rather than in a less degree after the excitability of the vegetable had been highly increased by a sudden abstraction of that stimulus in a previous extreme of cold, in a manner analogous to the sudden application of heat to a frozen limb? Here it may not be improper to state my own observations," he continues, "on the temperature of the atmosphere about the time when I conceive the injury began to take place. Travelling in the afternoon on the 11th of July, 1804, I found it so extremely cold as to induce a belief that the thermometer would have been near the freezing point in the shade. I had not the opportunity to ascertain the fact; but to my feelings it was a change unusually severe. The day following was much warmer, and for some days after the heat kept increasing: on the 16th, 17th, and 18th it was intolerably hot. Within about a week from that time the disease made its appearance. In my recollection, the wind during the cold was easterly, or towards the north. I am also much disposed to believe that the long drought previous to the cold was favorable to the effect."
Weakness, whether produced by internal or by external injury, appears to be the pre-disposing cause which favours the attachment and growth, or at least which incapacitates the plant from repelling the attachment and growth of parasitic fungi. Some circumstances, however, are wanting to make this case complete and satisfactory of the hypothesis which Mr. Egremont endeavours to establish, namely, that the previous disease does not arise from any rupture of the tumid vessels, which is a cause purely mechanical, but that it arises from the too rapid and sudden return of a stimulus previously abstracted from the plant, which is a cause entirely physical.
In the first place, Mr. Egremont draws his inference respecting the effect of a return of abstracted stimulus, without the least knowledge of the antecedent state of the plants—without being at all prepared to state that the vessels had not in fact been injured, perhaps ruptured by frost. A long previous drought certainly leads us to infer, that the plants were not in a very succulent state:
