337 
Sources of Nitrogen in Plants. 
that the root and fungus may be regarded as symbiotically 
related one to another. Of course this paves the way to a 
still wider definition of the idea Mycorhiza, and a concomi- 
tant risk of vagueness ; in fact, Frank has himself had to go 
much further, as will be seen from what follows. 
Frank’s second type of endotrophic Mycorhiza is that of 
orchids. It has long been known that the roots and 
rhizomes of exotic and native orchids contain hyphae, which 
live in the cells of the cortex. In 1886 Wahrlich 1 carried 
out a masterly investigation of the subject, along the well- 
known thorough lines for which the Strassburg laboratory 
is so celebrated, and showed that the fungus in question is 
a Nectria. Reference must be made to Wahrlich’s paper 
for details. He examined more than 500 species and all 
had the fungus.. Aerial roots are infected as well as others. 
The fungus only affects spots here and there, its hyphae 
coiling themselves up in certain cells into knots, which as 
a rule only partly fill the cell and do not destroy the proto- 
plasm but cause the cell to enlarge. 
Frank lays stress on the following points: (1) The proto- 
plasm of the cell and the fungus live together, ‘ without the 
former being parasitically affected or its vital phenomena 
disturbed.’ 
This can only be an assumption, and the impression I 
gather from the study of what is known of this orchid-fungus 
is in favour of the view that the fungus does disturb or 
‘ parasitically affect ’ the protoplasm of the cell, and that 
an outward and visible sign of some such action exists in 
the hypertrophy of the cells affected, and in the turning 
yellow of the chlorophyll-grains 2 ; moreover, as Frank him- 
self points out, the nucleus of the affected cell is larger. 
The conclusion that the fungus does not act as a ruthless 
parasite is warranted by the facts ; but not so the conclusion 
that the hyphae do not stimulate the cells to increased meta- 
bolic activity. 
1 Bot. Zeit., 1886, pp. 481-499. 
2 Wahrlich, 1 . c. p. 484. 
