39° 



LECTURE XXIV. 



quantities are produced. Moreover, the action of this ferment at any time need only 

 be exercised close to the surface of the growing end of the fungus-filament, so that 

 at distances measurably removed no further decomposition of the substratum follows. 

 This at least explains how it is possible for the fungus-filaments to bore smooth 

 holes (which they completely fill up) through the cuticularised epidermis of living 

 plants, and through lignified cell-walls, and even through starch-grains. Evidently 

 this fermenting action of a fungus-filament is to be compared with those which I 

 have already described (p. 344, Fig. 236) in the germinating Date. As the delicate 

 soft haustorium of the first leaf of the seedling there grows into the hard mass of 

 endosperm, because it dissolves and absorbs the hard cellulose as well as the proteid 

 substances and fat of the endosperm close to its surface, by means of its ferment action, 

 so, too, we may suppose the fungus-filaments penetrate into their solid substratum, 

 which is insoluble in mere water. 



That the ferment-fungi also exert ferment-actions on their substratum is shown 

 in the first place by the invertive action of Yeast, since it converts cane-sugar into 

 glucose, and we may certainly assume that in the nutrition proper of all Fungi which 

 have a destructive action on their substratum, ferment-actions first take place by 

 means of vi'hich a portion of the substratum is brought into a form capable of 

 nourishing the Fungus ; besides this ferment action, fermentation proper then comes 

 into play, by means of which the substratum (occasionally to the injury of the Fungus 

 itself) is destroyed. The peculiar behaviour of Peronospora {Phytophthord) infestans, 

 which nourishes itself for months in the tubers and green shoots of the Potato without 

 causing injury, and finally, as the fruit-bearing organs develope, rapidly extends and 

 kills and destroys the nourishing tissue, demonstrates that in this case the same 

 Fungus in different periods of life sometimes exerts ferment-actions and sometimes 

 promotes fermentation on its Substratum. In contrast to th6 destructive action which 

 the Fungi of fermentation and putrefaction in the widest sense exert on their sub- 

 stratum, we find, however, a large group of Fungi abounding in species, which, on the 

 contrary, influence their living substratum favourably, even promoting their activity 

 in order the better to make use of them. This is the case with the Lichens ^ It 



' With respect to the history of the discovery of the true nature of Lichens, De Bary expresses 

 himself as follows in his lecture, ' Ober die Erscheinung der Symbiose'' (Strasburg, 18791, p. 17 : — 

 ' From their (the gonidia) constant occurrence in every Lichen there has long been not the slightest 

 doubt that they are organs of these otherwise Fungus-like plants :' their resetnblance to Algae was 

 also obvious, and the Lichens were therefore regarded as a group standing between Algae and Fungi. 

 These views obtained a particularly firm basis by SchwendenCr's thorough studies into the structure of 

 the Lichen- thallus, from which it seemed to result that the gonidiaarise as small branches or as the ends 

 of branches of the filaments devoid of chlorophyll. However, several matters still remained obscure. 

 In particular, the first origin of the gonidia-bearing thallus from the typical reproductive organs, the 

 spores ; since when these were sown, and the sowing kept under rigorous control, there always arose on 

 germination only those temporary Fungus plantlets referred to in the text, and no gonidia-bearing 

 Lichen-thallus, and in rare cases where such were obtained on sowing it was not clear whence the 

 gonidia had come. On the g;round of these and similar considerations I first expressed iii 1866 for 

 certain Lichens the hypothesis (based on extensive unpublished investigations) that they might 

 possibly result from the union of a certain definite Fungus with an Alga. The extension of this 

 hypothesis to all Lichens was not admissible by the then existing researches, particularly Schwen- 

 dener's. Afterwards, when, by the works of Famintzin anil Baranetzki especially, the probability 

 became more and more prominent that the so-called gonidia are identical with Algae which occur 

 independently, Schwendener was enabled by his later investigatiofls to establish the theory summed 



