GROWTH OF PARASITIC FUNGI. 



roots or by the leaves, it is probable that both the early and late varieties were inoculated 

 with the germs at the same time, but in one the conditions were favorable for their de- 

 velopment while in the other the}^ were not favorable, and the difference must obviously have 

 been owing to some internal cause as both were subject to the same external influences. 



It has been commonly observed that varieties of the potato are usually attacked as they 

 approach maturity, as if a certain cessation or decline of the growth of the plant favored 

 the growth of the parasite; varieties of the potato are not however invariably mildewed 

 at a certain stage, or when the plants have nearly perfected their tubers, neither is wheat 

 and other grain crops always, though generally, blighted by rust or mildew when the 

 plants are in ear. The vitality of an annual plant like wheat, or of the potato, whose 

 stems endure but for a season, is probably more energetic at the commencement of their 

 growth than when they have nearly performed their functions, and they may in conse- 

 quence be better able to resist the action of ordinary external influences at that time than 

 afterwards. If this be so, then it is not unlikely that different varieties of our cultivated 

 plants growing under the same circumstances may ofier greater resistance to attacks of 

 fungi than others seeing that they possess different degrees of constitutional vigor. This 

 is a point in the inquiry of considerable interest, whether according to the vigor or con- 

 trolling power of the vital princij^le of a plant, so will be its power of contracting the 

 action of ordinary external influences and its comparative freedom from disease arising 

 from attacks of fungi. 



Many of our garden plants are propagated by extension, that is by buds, cuttings, lay- 

 ers or roots instead of by seeds, — and notwithstanding the " tale of woe" revealed to 

 your correspondent, Mr. Marshall, by the gallant old apple tree, which in spite of its age 

 and infirmities persisted in believing that it retained the vigor of youth, I may be per- 

 mitted to say that the evidence of apple trees and other plants seems to me to afford sub- 

 stantial grounds for coinciding with the views advanced by Andrew Knight, that each plant 

 propagated by extension has a limited duration, that it cannot by any known means be 

 continued equally healthy and vigorous forever; but that sooner or later the progeny will 

 gradually decline in vigor become unhealthy and unproductive, not suited to the purposes 

 of the cultivator and consequently extinct. This also suggests an interesting branch of 

 the inquiry, whether as a variety declines in vigor it becomes more subject generally to 

 attacks of fungi, or if an attack is more virulent and damaging than on younger and more 

 vigorous varieties, growing under the same circumstances. 



The hop is generally propagated by cuttings, and a writer who has raised plants from 

 seed, in consequence of his old plants dying off, and the hops becoming small, and of a 

 bad color, said, " We write from experience; having raised very many hop plants from 

 seed, we have found them to be much more energetic and vigorous than those which have 

 been raised from cuttings, their luxuriant growth enabling them to withstand the effect 

 of blight." It is well known that some varieties of the potato have suffered much less 

 from the blight than others, while some varieties recently raised from seeds, in the United 

 States and Prussia, are said to have continued free from the disease, though growing near 

 to others which were infected. I believe the potato plant, considered in the mass or as a 

 species, to be in an unhealthy and degenerate condition; that it is, in fact, hcredeterarily 

 diseased, the accumulated result of maltreatment and neglect in the culture of the plant, 

 operating through successive generations. This, I believe, is the foundation or predispos- 

 ing cause of the disease; that the parasitic fungus, Botr3'tis infestans, is the immediate or 

 exciting cause, the observations of men so well qualified to observe with accuracy in mat- 

 ters where fungi are concerned, as Berkeley and Morren, have already placed beyond the 



