GROWTH OF PARASITIC FVNGI. 



reach of controversy, though possibly not of cavil. The unprecedented attack on the 

 potato, so much more extensive and virulent than the usual attacks of fungi on other cul- 

 tivated plants, T consider to be the result of its previously unhealthy condition. If these 

 views are well founded, then the question, What are the conditions required for the growth 

 and increase of parasitic fungi.' has a great practical bearing, if considered with reference 

 to this plant only, because if their growth and increase does not depend solely on atmos- 

 pheric influences, but is favored by the unhealthy state of the larger plant, then we may 

 reasonably hope, that by restoring the plant to its pristine vigor, with judicious selection, 

 through several successive generations of seedlings, with improved culture, we shall ob- 

 tain varieties which, when propagated by divisions of the tuber, will, for a time at least, 

 resist the attacks of their parasite, just as wheat, turneps and other crops annually raised 

 from seeds, now do; further than this we are not justified in anticipating. What we know 

 of the attacks of fungi on other crops, forbids the hope now that the parasite of the pota- 

 to is established in this country, (for several facts seem to indicate that it is a recent in- 

 troduction to North America and Europe,) that our crops will ever be entirely free from 

 mildew when the potato is restored to the highest rate of health which it is capable of at- 

 taining. And it is equally vain to expect, as many have done, that the vigor of the plant 

 can be restored by one generation of seedlings. The progeny of unhealthy and degene- 

 rate parents cannot reasonably be expected to be perfectly healthy and hardy. The result 

 of upwards of a century of mismanagement, cannot be entirely obliterated by the first 

 step in the right direction. 



The fact that pear seedlings are .attacked by their parasite, may seem to many a serious 

 objection to the conclusion I am induced to arrive at; but as plants annually raised from 

 seeds are not exempt, I do not see why young plants of the pear should offer greater re- 

 sistance, when exposed to influences which afl'ect their health. I think it may also be 

 questioned whether a plant whose average duration is supposed to be about 400 years, is 

 not more susceptible of injury, or more easily aifected by adverse atmospheric influences 

 in the first few years of its existence, than when in the prime of its life. This seems to 

 be the case with the Osage orange, at least,— but possibly this can hardly be considered 

 a case i« point, inasmuch as the difference in the power to withstand the severity of our 

 winters, may be owing to the more concrete state in which I suspect the sap is deposited 

 in the tissue of the wood during winter, in mature, as compared with young plants. A 

 further question arises here, whether woody plants, when once attacked, do not afterwards 

 become more under the control of their parasite. The first appearance of the fungus on 

 Mr. Knight's pear trees, caused him no alarm, it being confined to spots on the leaves, but 

 in after years the young wood also became affected, and perished. 



The way in which parasitic fungi injure plants, is not, I believe, understood generally. 

 They do not grow on the surface merely, of leaves, prevent perspiration, and thus cause 

 death. They spring from the living tissue and destroy it. What we see of the plant ex- 

 ternally, is merely the fructification — the plant itself, composed of minute, thread-like fila- 

 ments, spawn or mycelium, as it is usually called, is concealed in the tissue. Now the 

 contents of the cells of plants are various, as researches recently made by the aid of che- 

 mical reagents indicate; the mycelium of fungi has the power of piercing the cell walls. 

 By thus rupturing the tissue the contents are set free, they are no longer under the control 

 of vitality, hence chemical action ensues, putrefaction of the part attacked follows, morbid 

 matter is thus generated, which circulates with the fluids of the plant, and gives rise to 

 unhealthy action 

 this matter as on so many other points, a close analogy seems to subsist between the 



