362 



SCIENCE. 



[Vol. XIV. No. 356 



In tl e October bulletin of the Hatch Experiment Station of the 

 Massachusetts Agricultural Coll'=ge. Professor James Ellis Huin- 

 phrey, professor of vegetable physiology, suminarizes the results of 

 his investigations on the subject. It is doubtless true that to the 

 average reader the ternn "fungus" carries with it no definite idea. 

 This is due partly to the newness of the popular use of the term 

 and the meagreness of generally accessible sources of information 

 concerning the fungi, and partly to the inherent difficulty and 

 technicality of the subject. To obtain a clear notion of organisms 

 so small as to be barely recognizable by the naked eye, and requir- 

 ing high powers of the microscope for their study, yet with such 

 apparently disproportionate capacities for mischief, is not easy. It 

 is for this very reason all the more important, that, in a discussion 

 of fungous diseases intended for popular information, an attempt 

 should be made at the outset to remove, so far as may be, this 

 fundamental difficulty. 



In the first place, then, a fungus is a plant — as truly and essen- 

 tially a plant as the corn-stalk or rose-bush on which it grows. 

 Yet it is not only much smaller, but also much simpler, than these. 

 While the plant- body of the corn or rose shows much specializa- 

 tion of structure, having the various vegetative functions of the 

 plant performed by distinct organs (the root, stem, and leaves), 

 very many plants show no such specialization, but have all their 

 vegetative functions performed by the whole plant-body, which 

 then needs no variety of organs. Of the latter class of plants are 

 the rockweeds and sea-mosses, the fresh-water pond-scums and 

 the fungi, which are obviously much simpler and more primitive 

 plants than those with roots, stems, and leaves. In all true fungi 

 the plant-body consists of numerous simple or branching white 

 threads which spread over the surface or through the substance of 

 the object on which the fungus grows. These threads constitute 

 the so-called " mycelium " of the fungus, and are comparable with 

 the more elaborate plant-body of other plants, since they perform 

 all its vegetative functions. 



Equally important with its own healthy growth is the provision 

 by any plant or animal for the perpetuation of its kind, and to this 

 end it develops organs of reproduction. In many of those plants 

 provided with root, stem, and leaf, these reproductive organs are 

 grouped into a structure called a flower; and such plants are 

 known as " flowering plants." They all produce, by the further 

 development of certain parts of their flowers, structures known as 

 " seeds," which can, under favorable conditions, develop into new 

 plants similar to that which produced them. 



Fungi do not produce flowers, and they vary greatly in their re- 

 production; but they all agree in producing bodies called "spores," 

 — much simpler than seeds, as would be e.xpected, but analogous 

 to seeds in their ability to develop, under favorable conditions, into 

 plants similar to those which produced them. These spores are 

 usually produced on special fruiting or reproductive threads, which 

 grow from the vegetative threads of the mycelium of the fungus. 

 The reproductive threads may remain separate, thus producing 

 their spores free in the air ; or they may become interlaced or con- 

 solidated into a complicated fruiting structure, on which the spores 

 are produced either superficially or in cavities, from which they 

 finally escape into the air. The spores of fungi, being so small 

 and light, are readily taken up and widely spread by currents of 

 air, and are easily carried by insects from plant to plant. In such 

 ways a fungous disease may spread from a single insignificant case 

 until it becomes epidemic over a large area. 



In the course of its life- cycle, the ordinary flowering plant passes 

 from the seed, through the seedling, to the adult plant, bearing 

 flowers and then seeds like that from which it grew>. Many of the 

 fungi, however, pass through a much more complex life-cycle, dur- 

 ing which a given fungus may produce several kinds of spores, and 

 assume several forms so unlike each other that they can be recog- 

 nized as different stages of the same plant only by careful, patient 

 cultivation and study. It is convenient to select some one stage 

 of such a variable fungus as its perfect or adult form, and it is natu- 

 ral and logical to regard as such that stage in which the fungus 

 shows the greatest elaboration of structure, while the simpler 

 stages through which it passes are commonly called " imperfect 

 forms. This tendency of fungi to variety in form, or " pleomor- 

 phism," as it is called, greatly increases the difficulty of their sturly, 



and complicates those problems which concern the successful com- 

 bating of fungous diseases. 



A question which very naturally suggests itself is, " Why do 

 fungi attack and cause diseases of other plants, instead of living 

 independently?" This question involves matters of the greatest 

 interest and of fundamental importance and significance. It is well 

 known that all " green " plants owe their characteristic color to the 

 presence of a definite pigment known as " leaf-green," or " chlorc- 

 phyl," which is so generally present among the higher plants, that 

 to most minds the very word " plant " carries with it the idea of 

 greenness. Now, the possession of chlorophyl is the pre-eminent 

 feature which gives to plants their all-important place in the econ- 

 omy of nature. No living thing can continue to live on inorganic 

 substances, but all require as food some of those materials of com- 

 paratively complex chemical composition known as organic sub- 

 stances. The materials furnished by the earth, the air, and water 

 are all of simple composition and unorganized ; but in leaf-greerv 

 we have the connecting link, the means of bridging the interval 

 between the inorganic and the organic. Professor Humphrey does 

 not discuss the process in detail. He thinks it sufficient for pres- 

 ent purposes to say, that, in Nature's laboratory of the leaf, some 

 of the simple constituents of air and water are combined, by the 

 action of leaf-green in the sunlight, into the complex organic com- 

 pounds which serve the plant as food. The chemistry of this re- 

 markable process is not well understood, but the commonest 

 permanent form in which these food- materia Is appear is that of 

 starch. 



Now, as was noticed above, the threads of the fungi are white, 

 uncolored ; that is, they contain no leaf-green : consequently the 

 fungi cannot elaborate their own food- material, but must obtain it, 

 ready elaborated, from some other source. Evidently the available 

 sources of organic food-supply fall under two heads, — living 

 organisms ; and dead organic matter, commonly decaying. On 

 this basis, the fungi may be divided into two classes, — those 

 which derive their nourishment from other living things, and those 

 which live on the remains of dead organisms. The latter, known 

 as " corpse plants " or "saprophytes," include the moulds, toad- 

 stools, and many other fungi ; but the first-named group is that 

 which at present is of interest, since it contains the various groups 

 mentioned at the beginning, which live on or in the bodies of other 

 living plants at their expense, and cause extreme weakening or 

 even the death of the affected plants. Such fungi are known as 

 "parasites,'' and the plants they attack are called their " hosts." 

 This distinction between saprophytic and parasitic fungi is a very 

 useful one ; but no sharp jine can be drawn between the two 

 groups, since some fungi seem to be able to live either as parasites 

 or as saprophytes, while it is probable that very many pleomorphic 

 fungi are parasites in some of their forms, and saprophytes in other 

 stages of their life-cycle. 



Finally, the interesting fact may be noted, that any given para- 

 sitic fungus is usually restricted in its capacity for harm to a single 

 host-plant or to a few closely related ones ; though, on the other 

 hand, closely related fungi may attack plants of widely different 

 relationships. Thus, the mildew of the lettuce and that of the 

 onion are very closely related fungi ; yet neither mildew can attack 

 the host-plant of the other, since the structural resemblances are 

 few and the relationship remote between the lettuce and the 

 onion. 



From the above facts may be derived a few iinportant principles 

 for guidance in attempts to avoid or check the ravages of fungi 

 among plants cultivated for use or beauty. Since the mycelium of 

 a parasitic fungus grows usually within the tissues of its host-plant, 

 it is too late to try remedies after a plant is once infected. It is 

 true that a few fungi are superficial in growth, and a treatment 

 may perhaps be found which shall destroy such parasites without 

 harm to the host ; but in most cases the aim must be to fortify ex- 

 posed plants against infection by the timely application of protec- 

 tive solutions or mixtures, which shall prevent the germination of 

 the spores which fall upon the plant so treated. Some progress 

 has been made in this direction, and some results have been reached 

 which justify hopes of ultimate general success in largely avoiding 

 the present enormous annual losses resulting from fungous dis- 

 easps. 



