334 



SCIENCE. 



[Vol. II., No. 31. 



■winds sweeji so as to keep the surface-water rough- 

 ened. 



The spread of epidemic diseases in plants. 



BY V,'. O. FAKLO'W OF CAMBKIDGE, MASS. 



In the case of animals it can be said, that, except- 

 ing the diseases attributed to bacteria, they are subject 

 to but few diseases caused by fungi. In the case of 

 plants, however, the greater part of the diseases to 

 which they are subject are caused by parasitic fungi; 

 excepting, of course, the injuries caused by insects, 

 which need hardly be considered in speaking of epi- 

 demic diseases. Most of the violent epidemic diseases 

 of plants are caused by fungi of the orders Uredineae, 

 rusts, and Peronosporeae, rots. Fortunately the spe- 

 cies of these orders attack only a single species of 

 host, or at most several species closely related botan- 

 ically ; so that, for instance, a rot which would attack 

 the potato would not probably attack the grape, 

 although it might be expected to attack the tomato, 

 which is botanically closely allied to the potato. As 

 might be expected, the most violent epidemics occur 

 duiliig, or just after, unusually wet periods. An 

 epidemic disease spreads either by the dispersion of 

 its spores through the air, or by the transportation 

 of the host-plant on which it is growing; the latter 

 being probably the means by which diseases are 

 carried across large bodies of water, as the Atlantic. 



With the introduction of food-plants from Europe 

 to this country come, of course, many of th fir parasitic 

 diseases. It should be noted, however, that the most 

 violent plant-epidemics of recent times have advanced 

 not fi'om east to west, but from west to east. The 

 best-known case is that of the potato-rot in 1845, and 

 since then the very accui-ately recorded case of the 

 grape-mildew, Peronospora viticola, has arisen. In 

 the first case, the disease is supposed to have reached 

 Europe from the west coast of South America, by way 

 of the United States. In the latter case, the grape- 

 ' mould, which is a native of North America, can, as I 

 showed by experiments in 1S76, be transferred to the 

 European vine ; and it was prophesied that the disease 

 would extend to Europe, and do more harm than with 

 us. The prophesy was very soon fulfilled, as you all 

 know. In the two diseases just mentioned, it is a 

 characteristic of the spores, that in germinating, in- 

 stead of giving off a filament, they discharge a number 

 of motile zoospores, each of which is capable of propa- 

 gating the disease. We have several other species of 

 Peronospora, which produce zoSspores, some of which 

 have apparently crosseil from America to Europe; 

 and there are others which, although common in this 

 country, have not yet appeared in Europe, although, 

 following the grape-mould, they may be expected to 

 appear there hereafter. Among these may be men- 

 tioned Peronospora Halstedii, which grows on com- 

 posites, and may later be found in Europe on the 

 Jerusalem artichoke. Professor Trelease has recently 

 found a Peronospora on Sicyos in Wisconsin, which 

 resembles the grape-mould in general appearance. 

 The germinatinn of the spores has not yet been ob- 

 served, but judging by analogy one would expect 

 them to produce zoospores. It would not be surpris- 



ing if the Peronospora on Sicyos should also be found 

 heieafter causing a disease of squashes or melons; 

 and its progress eastward might be expected as in 

 the cases previously cited. 



The speaker then referred to a uiodiBcation of the 

 spores sometimes observed in Peronospora. Mr. 

 Earle of Cobden, 111., collected species on Geranium 

 and Viola, where, instead of the usual blanching 

 spore-stalks, the spores were borne on the mycelium 

 close to the breathing-pores; the spores themselves 

 being very much larger than in the conuuon form. 

 A simil.ar monstrosity has been noted by Cornu in the 

 grape-mould. The specimens were collected by Mr. 

 Earle in April, and the speaker suggested that this 

 form of spores might perhaps be an adaptation to 

 the cold and wet weather of spring. The conditions 

 which produce the monstrous forms are worth con- 

 sidering by collectors. 



Of the diseases caused by Uredineae which have 

 advanced from west to east, the hollyhock-disease, 

 Puccinia malvacearum, is the best-known instance. 

 Its original home was probably Chili; but it spread 

 through Europe about ten years ago, not, however, by 

 way of this country, as was ju-obably the case with 

 the potato-rot. The diseases produced by fungi of 

 other orders, as Ascomycetes, do not spread with the 

 same rapidity as the rusts and rots. This is shown 

 by the black knot, which is so destructive in this 

 country to plums and some kinds of cherries. It is 

 a native of this country, and is found on most of our 

 wild species of Prunus, especially the choke-cherry, 

 a shrub which has been introduced into many places 

 in Europe. As yet, however, the black linot has not 

 made its appearance in Europe. 



The speaker then said that he 'had just found the 

 grape-mildew growing on the Virginia creeper (Am- 

 pelopsis quinquefolia) near Minneapolis. As this 

 plant is closely related to the vine, the occurrence of 

 the mildew might have been expected. In attempt- 

 ing to prevent the spread of the disease to countries 

 where it is now unknown, the discovery is of impor- 

 tance. It is evident, that, to prevent the spread of 

 the disease, the importation of Ampelopsis as well as 

 of grape-vines must be prohibited. 



Parallelism of structure of maize and sorghum 

 kernels. 



BY E. L. STUKTEVANT OF GENEVA, N.Y. 



If kernels of flint, pop, sweet, and Tuscarora maize 

 he split parallel to the germ, each race will be seen to 

 present a definite arrangement of structure. Thus, 

 the flint corn presents a germ surrounded by starchy 

 matter, and this in turn by a corneous envelope; in 

 the pop-corn proper, the germ is enclosed in the cor- 

 neous matter, the starchy matter being absent except 

 as the pop variety intrenches upon the flints; the 

 sweet corn has a similar structure to the pop, but the 

 corneous matter is translucent and wrinkled. 



By means of blackboard diagrams, the relative 

 arrangements were exhibited of the 'chit' or germ, 

 the corneous matter, and the starch, in the kernels 

 of the above-named varieties of maize and in sorghum. 



