230 APPENDIX. [No. XV.B. 



the insect makes its puncture, and breaks up the cellular tissue, or the 

 death may be remote from the part first attacked ; and remote death 

 resulting from the attacks of the aphides invariably kills the plant, 

 because the supply of nourishment becomes cut off from its upper part. 

 Death produced in this remote way often causes the potato-plants which 

 appear in a green, succulent condition to wither up in a few hours, the 

 communication between the stem and root being cut off by the action of 

 disease ; so that the total death of the plant may arise from the death of a 

 part necessary to the whole. Plants resist the attacks of aphides better 

 under some conditions than others. All wild plants resist better than 

 cultivated ones. A wild turnip in a field will flourish amid the ravages of 

 aphides when all the cultivated ones are destroyed ; and the same is true in 

 reference to the wild potato, and indeed all wild plants. The potato-plant, 

 as we cultivate it, is in an unnatural condition, differing from the wild or 

 natural plant in having great excess of tuber (an excess of cellular tissue 

 over fibre) and great deficiency of leaves. Wild plants, or plants in a con- 

 dition well calculated to develop fibre, well resist the attacks of aphides, 

 when highly-cultivated plants, or plants not under circumstances favour- 

 able to the formation of fibre, resist them badly. There is a particular 

 period in the growth of the potato-plant (as well as others) when the solid 

 material formed or elaborated in the leaf is most wanted. At that period 

 the plant becomes most liable to die from any injurious causes. In the 

 potato, the most critical time in its growth is when the supply of nourish- 

 ment contained or stored up in the old potato, or set, at its base is con- 

 sumed; then, if the organization of the leaf has been injured, its functions 

 are impaired, and when called on for that purpose it cannot give the 

 necessary healthy vital fluid for the nutriment of the plant and the deposit 

 of solid fibre, and it dies in consequence; or it exists in a debilitated 

 condition, forming imperfect tissue; therefore it may be stated that 

 plants are most injured by aphides at that period of their growth when 

 they are required to deposit most fibre. When from such causes the 

 tissue of the plant has once been rendered imperfect or diseased, all future 

 growths have a tendency to continue the diseased action and to form 

 unsound tissue. You all know familiarly the hereditary tendency to 

 disease that exists in families, which may pass from generation to genera- 

 tion, and thus the faults and imperfections of one are transmitted down- 

 ward to another. Apple-trees, rose-trees, &c., when once debilitated, have 

 been noticed to exhibit a return of such condition in all future growths 

 emanating from them, and it is so in the potato, &c. This, then, leads us 

 to deduce a law, that plants having their tissue damaged from aphides 

 propagate diseased tissue in all their future growths. Generally, if a 

 plant begins to perish it is soon cut down, indeed in a very few days, and 

 the influence of the hot sun often causes it to perish very greatly in a 

 single day. 



The death of the plant exercises also an important influence on the 

 aphis. When its supply of subsistence becomes diminished, it does not 

 remain to perish amid the famine itself has made, but the pupa of the 

 aphis casts its coat, and becomes the winged insect, prepared to fly away 

 and commit similar ravages elsewhere. Vast clouds of them rise together 

 from fields that have perished, often forming quite a mist in the atmo- 

 sphere. I have accounts of these vast clouds of insects being seen in 



