760 



THE TROPICAL AGRICULTURIST. 



April i, 1885. 



conditions of weather and of the Vine have for generations 

 been as favourable for the growth of this mildew as they 

 are new ; only one element has been wanting— the spores. 

 The hitter have finally crossed the ocean, in some way 

 carried by man, and now the rapid development proves 

 the suitableness of the existing conditions for growth, but 

 their inadequacy for original production. In the same 

 way the black rot of American Grapes can be traced to 

 infection from Europe, and the same lessons arrived at. 



A few years since a parasitic fungus previously known 

 iu South America, gained introduction to southern Europe 

 aud gradually spread over the continent, carrying destruc- 

 tion to the Hollyhock as it went. Beaching England its 

 ravages were especially marked, since considerable promin- 

 ence is given this popular flowering plant. The Hollyhock 

 is closely allied to the (lotton plant, and as these fungi 

 sometimes grow on botanically related species there was 

 cause for apprehension, lest the parasite should be again, 

 transported across the Atlantic and bring serious trouble to 

 our sisterhood of Southern States. The watery bearer 

 was actually passed, but fortunately the Cotton did not 

 prove susceptible to its withering effects. 



The primitive origin of these species of fungi we will 

 not discuss. They in some way, at some time, came into 

 existence, and in the same manner that other species of 

 living beings, not excluding man, were originally produced. 

 Evolution has been studied, and in its light species are 

 said to be transformations of previously existing species ; 

 but this does not in any practical sense alfect the 

 foregoing, because the process is reckoned by centuries 

 and eons, not by years or the generations of men. 



It is possible that change of habit sometimes occurs 

 to such an extent that a fungus species not formerly 

 capable of growing on a certain host species, becomes 

 adapted to the latter; but nothing of this kmd has been 

 definitely observed. There is, however, the widest variation 

 among the species of parasitic fungi as to the limits of 

 their restriction to certain host plants. Many are found 

 only ou one species; very few (except such as have a 

 peculiar alternation of habitat after the manner of many 

 animal parasites) grow on plants belonging to different 

 botanical families. Yet a few affect many species of 

 flowering plants, even sometimes those of quite different 

 orders. In the latter cases there is usually more or less 

 difference in the vigour and appearance of the fungus ou 

 the different hosts; so that it is not easy to decide by 

 form and appearance alone whether a certain parasite on 

 a certain nourishing plant is, or is not, a distinct species, 

 or only a modified condition of something kuowu else- 

 where. ' Artificial cultures, by transferring the spores from 

 plant to plant aud watching their development, are the 

 only criteria when such doubts occur ; and this is entirely 

 feasible, though it requires much care and skill to secure 

 reliable results. The so-called black rot of Grapes is caused 

 by a minute fungus parasite of low and simple organiz- 

 ation affecting the young stems and leaves (petioles and 

 veins), as well as the fruit. There is also a disease of the 

 canes and leaves (petioles and veins) of Black Cap Hasp- 

 berries and Blackberries caused by a little fungus so sim- 

 ilar in every way as seen under the microscope that one 

 is inclined to pronounce them identical; but cultures prove 

 them distinct— a point of considerable practical importance. 



GERMINATION AND PENETRATION. 



The next thing deemed of most importance is to fully 

 comprehend that these parasites always germinate outside 

 the plant tissues and gain entrance, if at all, only by 

 mechanically penetrating the epidermis or other surface 

 coat. An unsubstantiated opinion too commonly prevails 

 that in some way the spores may be taken up by the 

 roots with water and carried with the latter to any part 

 of the plant. This assumption is founded upon a mis- 

 conception of the manner that plants take water from the 

 soil, and of the way it traverses the plant tissues. It is 

 true there are in most plants elongated ducts or tube-like 

 vessels, the open cavity of which is sometimes large 

 enough to be seen in cross section by the unaided eye, 

 and large enough to pass many fungus spores; but these 

 cannot, in any just sense, be compared to the arteries 

 and veins of animals. The truth is, when there is any 

 considerable movement of the watery fluids in plants, 

 these ducts are always filled with air, not with liquid 



material. If a sapling in full leaf, aud consequently in 

 its most active state as to the ascent of water to supply 

 the marvellous amounts transpired is cut, ami a portion 

 of the stem thrown into water, the latter will be sucked 

 into the tissues to the amount of 10 to 20 per cent of 

 the weight of the green stem, clearly showing that the 

 wood was not previously full. Other experiments and in- 

 vestigations prove that the water normally ascends (and 

 descends sometimes) through the substance of the cell 

 walls themselves, not through the cell cavities. Now, no 

 one is able to see with the best microscope ever made 

 the inter-molecular spaces in these cell walls, though 

 water, itself made up of solid molecules, passes through 

 them to gain entrance in the first place to the roots, and 

 through the millions of them iu its progress to the upper 

 portions of the plant. No fungus spore can pass such 

 filtering. The methods of freeing liquids from solid part- 

 icles practised by chemists are coarse and sorely inade- 

 quate compared with that in operation in plants. It is 

 absolutely impossible for any solid body large enough to 

 be seen at all by the highest microscopic powers in ex- 

 istence to pass through one such cell wall, much less 

 through the unnumbered myriads composing the tissues 

 of one of our ordinarily cultivated plants. All spores of 

 fungi rarely are less than one five-thousandth of an inch 

 in diameter, while a body less than one hundred-thou- 

 sandth of an inch can be seen aud studied, and as the 

 molecular openings through which water passes are still 

 less, probably much less, we may be certain that such 

 spores are effectually excluded from the circulation in the 

 plant tissues. Direct examination also proves that the 

 entrance of the fungus is affected by piercing the surface ; 

 the germinal tube accomplishing this by its power of ab- 

 sorbing the substance at the point of contact, or by reach- 

 ing and passing through a stomato. A thick epidermis is 

 often a complete safeguard against the former method, 

 this alone being sufficient to account for the immunity 

 of certain varieties from diseases which so nearly extermin- 

 ate others. It is scarcely possible that any parasitic fungus 

 should be able to make its way through the corky envelope 

 of tree trunks, &c, which we call bark, so long as the 

 latter is free from cracks or wounds. 



It must be remembered that to exist a fungus is as 

 dependent upon an organised structure as are other plants 

 and animals. It is not possible that this solid structure 

 can be dissolved and life continue ; it is not possible that 

 a fungus spore can be liquified, absorbed, and reorganised. 

 As well might a criminal think of reducing his body by 

 some chemical process to a liquid form in order to pass 

 through the merciless grating of his cell window, and live 

 afterward as a man. 



As a practical demonstration of the non-absorption and 

 non-circulation of fungus spores in the tissues of plants, 

 nothing can be more satisfactory than the results as known 

 of putting bunches of Grapes in paper bags to prevent 

 the rot so prevalent in our country. These diseases, for 

 there are several of them, are perfectly prevented by ex- 

 cluding the spores of the fuugi which produce them from 

 the fruit itself, though the rest of the Vine is not protected. 

 As a matter of fact, other parts of the Vine are parasitised 

 by the same depredators, aud sometimes serious injury 

 done ; but the effect is necessarily different from what it 

 is in the true pulp of the fruit. In these eases, and in 

 most cases, the mycelium, or root-like portion of fungi, 

 spreads but slightly from the first point of entrance, not 

 more than a few hundredths of an inch in the stems and 

 leaves, to a greater distance in the fruit. But a limited 

 number of species uniformly send their mycelium very 

 widely through the affected plant. Smut of AVheat shows 

 itself only in the head, but the fungus starts in the ger- 

 minating plantlet and traverses the whole length of the 

 straw. 



The conditions of germination are also important ele- 

 ments in a study of plant diseases due to fungi. The 

 spores are very simple in structure ; each consists of a 

 single cell formed of an enclosed mass of plastic substance 

 (protoplasm) around which are two coats, the inner thin 

 and flexible, the outer usually thicker and much less elastic. 

 In germination the outer is pierced or ciacked, and the 

 inner coat protrudes as a long tube containing still the 

 soft internal substance. This tube ia that which penetrates 

 the plant, becoming perhaps a hundred times as long as 



