482 



NATURE 



[September 16, 1897 



the cell-walls, and that the actual boring of the cell-walls is due 

 to a solvent enzyme secreted by the tip of the fungus, and in 

 clearly demonstrating this excretion of an enzyme capable of 

 dissolving cellulose carried a step further what was so far 

 known, principally from De Bary's researches, as to this 

 process. In 1892, Reinhardt showed that the tips of hyphce 

 curve over towards spores they are about to attack, and found 

 that sugar-gelatine of greater strength attracts them from the 

 same medium with a smaller proportion of sugar. 



Miyoshi then showed, in 1894, that if a leaf is injected with 

 a substance such as ammonium-chloride, dextrine, or cane- 

 sugar, all substances capable of exerting chemotropic attraction 

 on fungus-hyphns, and spores of a fungus then sown on it which 

 is not parasitic, the hyphte of the fungus penetrate the stomata 

 and behave exactly as if the fungus were a true parasite. 



This astounding result throws a clear light on many known 

 cases of fungi which are, as a rule, not parasitic, becoming so 

 when the host-plant is in an abnormal condition— ^.^. the 

 entry of species of Botrytis into living tissues when the weather 

 is cold and damp and the light dull ; the entry of Mucor into 

 various fruits, such as tomatoes, apples, pears, &c., when the 

 hyphae meet with a slight crack or wound, through which the 

 juices are exposed. Nay, I venture to suggest that it is even 

 exceedingly probable that the rapid infection of potato-leaves in 

 damp weather in July is not merely traceable to the favouring 

 effect of the moisture on the fungus, but that the state of super- 

 saturation of the cell-walls of the potato leaf, the tissues of 

 which are now unduly filled with water and dissolved sugars, 

 &c., owing to the dull light and diminished transpiration, is 

 the primary factor which determines the easy victory of the 

 parasite, and I suggested some time ago that the suppressed 

 life of Ustilaginea , in the stems of grasses, is due to the want 

 of particular carbo-hydrates in the vegetative tissues there, but 

 which are present in the grain. 



Miyoshi, in 1895, carried to proof the demonstration that a 

 fungus-hypha is really so attracted by substances on the other 

 side of a membrane, and that its tip pierces the latter ; for the 

 hyphse were made to grow through films of artificial cellulose, 

 of collodion, of cellulose impregnated with paraffin, of parch- 

 ment-paper, cork, wood, and even the chitinous coat of an 

 insect, simply by placing the intact films on gelatine impreg- 

 nated with the attracting substance, and laying the spores on 

 the opposite side of the membrane. 



Hyphse so separated by similar membranes from gelatine to 

 which the attracting substance was not added, did not pierce 

 the membranes, whence we may conclude that it is really the 

 substance referred to which incites the hyphre to penetration. 



Now, obviously, this is a point of the highest importance in 

 the theory of parasitism and parasitic diseases, because it 

 suggests at once that in the varying conditions of the cells, the 

 contents of which are separated only by membranous walls 

 from the fungus-hyphse, whose entrance means ruin and de- 

 struction, there may be found circumstances which sometimes 

 favour and sometimes disfavour the entrance of the hyphse ; and 

 it is at least a remarkable fact that some of the substances which 

 experiments prove to be highly attractive to such hyphse — e.g. 

 sugars, the sap of plums, phosphates, nitrates, &c. — are just the 

 substances found in plants, and the discovery that the action 

 depends on the nature of the substance as well as on the kind 

 of fungus, and is affected by its concentration, the temperature, 

 and other circumstances, only confirms us in this idea. 



Moreover, there are substances which repel, instead of attract- 

 ing the hyphse. 



Is it not, then, natural to conclude that the differences in 

 behaviour of different parasites towards different host-plants, 

 and towards the same host-plant under different conditions, 

 probably depend on the chemotropic irritability of the hyph* 

 towards the substances formed in the cells on the other side of 

 the membranous cell-walls ? And when, as often happens, the 

 effusion of substances such as the cells contain to the exterior is 

 facilitated by over-distension and super-saturation, or by actual 

 wounds, we cannot be surprised at the consequences when a 

 fungus, hitherto unable to enter the plant, suddenly does so. 



In spite of all the progress made towards an explanation of 

 the origin and course of an epidemic of rust, however, one serious 

 inconsistency has always puzzled men who have worked with it 

 in the open and on a large scale. This inconsistency concerns 

 the outbreaks of epidemics over large areas, at periods, and 

 within intervals, which do not agree with the weather records 

 and the described biological facts. We know, speaking generally, 



the conditions of germination of the spores, we know how long 

 infection requires, and the latent period is known : we know 

 much as to the conditions which favour or disfavour the fungus 

 mycelium in the tissues, and, nevertheless, an outbreak of disease 

 over large areas sometimes occurs under conditions which appear 

 quite inconsistent with this knowledge. 



During his six years' study of the wheat rusts Eriksson was so 

 impressed with these difficulties that he has lately committed 

 himself to an hypothesis which may perhaps crystallise the ideas 

 which have floated in the minds of several who have been puzzled 

 by these matters. 



The facts which seem to have finally impelled Eriksson to his 

 hypothesis were those of the distribution of the wild rusts and 

 grasses. Having learnt which grasses could infect the wheat, 

 oat, barley, and rye respectively, he found cases of epidemics 

 occurring where it was impossible to fit in the facts with the view 

 that spores had been transferred from these grasses within the 

 period required for infection and development of the disease 

 spots. Again, seasons occurred when all the conditions pointed 

 to the probability of a serious outbreak of rust, and no such 

 epidemic occurred. Further, experiments were made in which 

 cereals of varieties known to be susceptible to given rusts were 

 planted in close vicinity to grasses infected with such rusts, and, 

 nevertheless, in seasons eminently suitable for the outbreak of 

 this particular rust on these particular cereals none appeared, or 

 so little that it was impossible to explain the outbreaks of this 

 same rust on this cereal elsewhere, during that season, as due to 

 direct infection from the surrounding grasses. 



More and more it became evident that the infective capacity 

 of the rusted grasses is small, and confined to restricted areas, 

 and that the outbreaks in certain seasons — rust-years — must be 

 due to something other than wind-borne spores distributed by 

 gales over the district. 



Three hypotheses can be suggested to account for the non- 

 spreading of the disease on to susceptible cereals — ( i ) Indisposi- 

 tion to germinate on the part of the spores ; (2) unfavourable 

 weather for germination ; (3) some structural peculiarities of the 

 lea-ves on which the spores fell, of such a nature that infection 

 was prevented. 



The results of many experiments showed that, as matter of 

 fact, the spores are often very obstinate, and refuse to germinate 

 even when the weather is apparently favourable, and Eriksson 

 discovered during these experiments that cooling the ripe spores 

 on ice increased their germinating power. Neither of the other 

 two hypotheses mentioned could be brought into agreement with 

 the results, however. 



The conclusion was thus arrived at that an outbreak of rust 

 cannot always be referred directly to the normal germination 

 and infection of wind-borne spores from neighbouring centres of 

 infection. 



In some patches of extremely susceptible cereals, the disease 

 appeared simultaneously on plants isolated from all perceptible 

 sources of infection, and on plants not thus protected ; the date 

 of outbreak in these cases — reckoned from the sowing of the 

 grain — was far too late to be explained by direct infection from 

 spores on the soil, or on the grain sown. Experiments demon- 

 strated that if such spores had been there, and germinal tubes 

 formed as usual, the disease would have shown itself much 

 earlier. 



These and numerous other inconsistencies drove Eriksson to 

 look for an "internal source of infection," in spite of the im- 

 probability of any such existing, and of its apparent incompati- 

 bility with scientific theory since De Bary's time. 



Two methods were pursued. In one each plant of the cereal 

 was enclosed from the beginning in a long glass tube, stuffed 

 with cotton-wool above and below, and so carefully protected 

 against infection from wind-blown spores that we may accept 

 forthwith the improbability of such infection. 



Notwithstanding these precautions, the cereal was rusted at 



the same time as its unprotected neighbours, and equally badly. 



Granting the accuracy of the experiments, only two explana- 



i tions seem to suggest themselves. Either (i) winter-spores 



! attached to the grain had germinated and infected the young 



I seedling — a not impossible event, since several observers have 



found spore-bearing mycelia in the pericarp of the ripe grains, 



j and we know these spores can conserve their germinating power 



j for months ; or (2) the infective material had been handed down 



I to the embryo from the parent plant — an almost inconceivable 



! hypothesis. 



[ To answer this question Eriksson protected his seed-plants 



NO. 1455, VOL. 56] 



