SYMBIOSIS 241 



tions as well (compare Fig. 44, /// and IV). Finally, the indigestible portion 

 of the fungal mass aggregates into the centre of the cell (V), where it becomes 

 surrounded by a membrane. According to SHIBATA, there is only one slight 

 difference between Psilotum and Podocarpus ; the cells of the Podocarpus 

 nodules are densely filled with hyphae, and these are digested. Since the 

 assimilation of free nitrogen very probably takes place in Podocarpus, we shall 

 not be far wrong in interpreting the mycorhizal condition in other plants in 

 the same way. [TERNETZ (1904) has managed to isolate a fungus from the 

 roots of Ericaceae, which is in all probability the one concerned in the 

 formation of the mycorhiza, and which is the active agent in assimilating 

 the atmospheric nitrogen.] The knowledge we have gained as to the pheno- 

 menon in Neottia and Psilotum renders the various relations of Bacteria to 

 Leguminosae all the more intelligible (compare p. 239). 



Accepting this interpretation of endotrophic mycorhiza, the question 

 comes to be whether a combination of two non-chlorophylliferous organisms, 

 e. g. Neottia and a fungus, can be termed a case of symbiosis, since it is not 

 readily comprehensible wherein the reciprocity can lie ; our knowledge of the 

 nutritive relations is still too inadequate, and hence we may postpone further 

 discussion of the question. If the Phanerogams possessing mycorhiza be 

 green and can therefore assimilate carbon, it may be assumed that the 

 activities of the two symbionts are so regulated that the fungus collects 

 the nitrogen and the higher plant the carbon. 



At the same time there may be another explanation of this association, 

 other than the fixation of atmospheric nitrogen, viz. that the higher plants 

 are peptone or asparagin organisms, and that the duty of the fungus is to 

 manufacture these nitrogenous compounds out of humus. Possibly, however, 

 the fungus aids in the absorption of materials of the ash, and does not supply 

 the needs of the higher plant for nitrogen at all (STAHL, 1900). Fungi make 

 very heavy demands on such materials, and since they collect these very 

 rapidly, they are vigorous competitors with Phanerogams which work more 

 slowly on soils poor in nutritive salts. Higher plants are able to grow far 

 better in humus which has been deprived of the Fungi naturally present. As 

 long as these Fungi are present the Phanerogams exhibit all the evidences of 

 ' mineral starvation '. A mycorhizal union occurs especially in such plants as 

 live in humus, or for other reasons exhibit feeble inflow of minerals (e. g. weak 

 transpiration). Hence STAHL assumes that these plants make the Fungi contri- 

 bute to their wants in that respect, turning antagonistic neighbours into efficient 

 assistants. The part played by the higher plant so far as carbohydrate is con- 

 cerned, becomes intelligible on this view. On the other hand, STAHL' s hypo- 

 thesis appears to us to be subject to criticism, in that the fungus lives in 

 most cases quite in the interior of the root, and hence cannot be in a very 

 suitable position to aid in absorbing nutritive salts from the soil. The function 

 of the fungus, however, according to STAHL, consists not merely in the 

 absorption of the nutritive salts from the soil, but also in their transforma- 

 tion, so that the other member of the symbiotic union may receive the 

 products of assimilation ready made. STAHL comes to this conclusion from 

 noting that the majority of ' mycotrophic ' plants do not contain in their 

 tissues certain waste bodies, such as calcium oxalate, which are associated 

 with the assimilation of nutritive salts (comp. pp. 141 and 197). 



We must now glance at ectotrophic mycorhiza. This form of mycorhiza was 

 first drawn attention to by KAMIENSKI (1881) in Monotropa, and soon afterwards 

 FRANK demonstrated its occurrence in a large number of our forest trees (Cupu- 

 liferae, Betulaceae, Coniferae). The Fungi apparently members of the Agaricinae 

 and Tuberaceae generally do not enter in this case into the cells of the roots, 

 but form a densely-woven layer covering the root, not even leaving the grow- 

 ing points free. Here and there single fungus cells enter in between the super- 



