THE NUTRITION OF HETEROTROPHIC PLANTS 183 



isms takes place (which we will consider in greater detail later on) which fre- 

 quently leads to the formation of humus. Humus again forms the home of 

 a large number of Fungi and also of Phanerogams, which, like Monotropa and 

 Neottia, are heterotrophic in their mode of nutrition owing to their possessing 

 no chlorophyll. Leaving on one side for the moment the Phanerogams, whose 

 complicated nutritive relations we shall consider later, there are numberless 

 Basidiomycetes at least, of which it may be said that they obtain all the organic 

 materials they require from humus, although what precisely these substances 

 consist in we are quite ignorant. The peculiar humus substances which are 

 soluble in alkalis cannot, according to Reinitzer (1900), at least in most 

 Fungi, act as a source of carbon, although they may serve as a source of 

 nitrogen (compare Nikitinski, 1902) ; Reinitzer, who experimented with 

 Penicilliiim, holds that possibly certain ' specialists ' may obtain their supply 

 of carbon from humins. 



We do not know precisely what other organic substances occur in humus 

 besides humins, and although we are unable to extract any useful nutritive 

 materials from it by means of ordinary chemical media, it is quite likely that 

 many of the Fungi are able to do so by excreting enzymes with dissolving 

 capacities. These enzymes are among those already studied, viz. diastases 

 and sugar -splitting enzymes, also cytase and protease. (Many other enzymes 

 occur in Monilia ; compare Went, 1901). On the enzymes occurring in Fungi 

 a number of interesting observations have been made, two of which only we 

 shall refer to here. In the higher plants we found cytases occurring as 

 a rule only when required to dissolve reserve cellulose ; the cellulose of ordi- 

 nary cell-walls in the presence of these enzymes remains as originally formed 

 completely untouched ; it is not dissolved and reabsorbed before the fall of 

 the leaf, and thus when the leaves and branches fall a large amount of organic 

 material is lost. In many Fungi, however, not merely those which live as 

 ' specialists ' on wood, such as Merulius lachrymans, the dreaded ' dry rot ', and 

 other wood destroyers, but also in Fungi in general, the power of dissolving cell- 

 walls has been definitely proved to exist. This capacity is obviously developed 

 in many cases only to enable the fungus to enter the interior of the cell ; the 

 dissolution of the cell-wall is subsidiary to the chief object, viz. to reach the 

 cell contents, starch, &c. In other cases the fungus apparently lives chiefly on 

 the cellulose, and is even able (Czapek, 1899) to make use of lignified walls, 

 effecting, by means of a special enzyme, a splitting off of the cellulose from 

 the etherial substance combined with it (hadromal, compare p. 70). The 

 cellulose is assimilated, but the hadromal remains unused. In this capacity 

 possessed by these Fungi we have one mode of destroying cellulose in nature ; 

 in Lecture XVII we shall have to speak of another method of achieving the 

 same result. Were it not for such decompositions the surface of the earth 

 would soon be entirely covered with thick layers of cellulose. 



Another observation which may be referred to deals with the excretion of 

 diastase. So far as seedlings are concerned a controversy still exists as to 

 whether diastase may be excreted from living cells, since it is often assumed 

 that it cannot pass through the cell-wall (compare p. 156, Lecture XII). It 

 cannot be doubted that in the case of Fungi and Bacteria such an exudation 

 of diastase through the cell-wall does take place. Wortmann (1882), and 

 especially Pfeffer (1896) and Katz (1898), have shown that the production 

 of diastase is not a constant characteristic of certain Fungi, but that it can 

 be induced or inhibited by external conditions. ■ In the abundant presence 

 of various sugars but not of all suitable nutritive sources of carbon, no 

 diastase is formed; in Penicillium, for example, a 2 per cent, solution 

 of sugar is sufficient to inhibit its formation. Bacterium megatherium behaves 

 in a similar manner, while in the case of Aspergillus a 30 per cent, solution of 

 sugar only retards, but does not inhibit, the formation of diastase. The 



