854 REPORT— 1897. 



trings its tip out of tbe previous direction, and De Bary even threw out the hint 

 that this attraction might he due to some chemical suhstance excreted by the host- 

 plant. I myself showed that the condition of the attacked plant affected the ease 

 ■with which the tube penetrates 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 hyphae 

 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-hyphse, and spores of a fungus then sown on it 

 which is not parasitic, the hyphse 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 — e.g., 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 hyphse 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 dis- 

 solved 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 Ustilaginece, in the stems of grasses, is 

 due to the want of particular carbohydrates 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 hj-phse were made to grow through films of artificial 

 cellulose, of collodion, of cellulose impregnated with paraffin, of parchment-paper, 

 cork, wood, and even the chitinous coat of an insect, simply by placing the intact 

 films on gelatine impregnated with the attracting substance, and laying the spores 

 on the opposite side of the membrane. 



Hyphas so separated by similar membranes from gelatine to which the 

 attracting substance Avas not added, did not pierce the membranes, whence we may 

 conclude that it is really the substance referred to which incites the hyphse 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 destruction, 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 hyphfe — 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 attracting the hyphse. 



Is it not, then, natural to conclude that tbe differences in behaviour of different 

 parasites towards different host-plants, and towards the same host-plant under 

 different condition.s, probably depend on the chemotropic irritability of the hyphse 

 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 



