Nov. 1897.] BOTANICAL SOCIETY OF EDINBURGH 73 



cold, and of drought and washing, the maintenance of an 

 equable temperature and an equable dampness, both of 

 which are favourable to the decay of vegetable matter, and 

 its conversion into humus, which, besides vegetable matter, 

 contains also the mineral substances most essential for the 

 nourishment of plants. But if Frank's observations are 

 thoroughly to be relied on, it would seem that humus has 

 become invested with a new interest, as the dwelling-place, 

 namely, of a fungus which assimilates what is most useful 

 as plant food and conveys it to the roots of the trees. 



So far from the mycorhiza being a parasite invading the 

 trees, it would rather seem that the trees themselves were 

 the parasites living on the fungus. 



In order that a true case of symbiosis may be established, 

 it would be necessary to show that the advantages enjoyed 

 by the trees from the association with the fungus were 

 reciprocated in some way. If the roots of the trees provided 

 a nesting place for the fungus where it was able to breed 

 or perform some function essential or advantageous to its 

 development, then it might fairly be called a case of sym- 

 biosis. It seems, however, that only the vegetative part of 

 this universally distributed fungus has as yet been found, 

 and it is not known whether the process of reproduction 

 takes place in the soil or in the roots of the trees, and, 

 until that is known, the truly symbiotic character of it 

 can be but imperfectly understood. It would seem, how- 

 ever, that its connection with the roots has this advantage, 

 that it puts it in a position where it grows immensely 

 more luxuriantly than it does when remaining unattached 

 in the soil. 



Since Frank's original paper was published, several 

 investigators have made observations on the subject, with 

 the result that the mycorhiza has been found aftecting 

 many other plants, notably Ericas and Orchids, and a 

 number of plants whose connection with the soil, or I 

 should rather say want of connection, has been a puzzle to 

 botanists, such as Paris quadrifolia and the Droseras, which 

 Schlicht, a pupil of Frank's, has been able to identify as 

 mycorhiza affecting internally the roots of a large number 

 of plants belonging to many natural orders. 



To return to the subject of the assimilation of the free 



