Rccyner.—Obligate Symbiosis in Callnna vulgaris. 127 
(p. 107), while nitrogen fixation has been claimed for five pycnidia-bearing 
Fungi isolated from members of Ericaceae, growing in pure culture outside 
the plant (26). The possibility of growing the artificially infected plant of 
Callnna in a nitrogen-free substratum awaits experimental proof. 
The only green plant for which has been described a like distribution 
of mycelium—not obviously pathogenic—in the tissues, combined with 
ovarial infection of the seeds, is the Darnel grass (Lolium temulentum). 
In this case, however, the plant does not form mycorrhiza, nor has 
it been established that the relation with the Fungus is an obligate one 
(47, 48). 
Some degree of symbiosis has been inferred, but the experiments of 
Hiltner (33) to establish nitrogen fixation for this Fungus are inconclusive. 
The case of the saprophytic Orchid Gastrodia elata , described recently 
by Kusano (49), is of interest, since there is dependence of the plant upon 
fungal infection for a part of the life-history only, coupled with a strictly 
limited distribution of the Fungus in the tissues. 
The higher symbiont in this curious relationship is a rootless sapro¬ 
phytic species of Orchid ; the fungal partner is Armillaria mellea (‘ Rhizo- 
morpha subterranea ’), a well-known facultative parasite. The association is 
an obligate one for the plant, in so far as the flowering stage is not reached 
unless ‘ mycorrhiza * is formed, but the vegetative period of the life-history 
is independent of infection, which takes place only occasionally in nature. 
It involves symbiosis, by means of a stem mycorrhiza, on the part of two 
heterotrophic plants, the fungal partner remaining apparently unmodified by 
its temporary association with the plant. 
The view is put forward by this author, that the first step towards the 
formation of mycorrhiza can here be recognized, involving a temporary 
modification of parasitic habit on the part of the Fungus concerned. 
It is evident, from a consideration of mycorrhiza in general, that it is 
still impossible to frame a definition which will include all the known cases. 
The theories of earlier observers implied a strict symbiosis with reciprocal 
advantages of an obvious kind. Later workers tend rather to regard the 
relation as primarily one of parasitism on the part of the Fungus, tolerated 
and often turned to account by the plant, or even become indispensable 
to it. 
Thus Gallaud (20) concludes that there is no ‘ symbiose harmonique ’ 
between plant and endophyte. The latter is only an internal saprophyte 
of a special kind which the plant cells can keep in check without preventing 
further development. 
Bernard holds similar views with regard to the Orchids: 
‘ A un point de vue theorique il resulte de ces constatations que l’etat 
dit de symbiose est en quelque sorte un etat de maladie grave et prolongee, 
intermediate entre Tetat des plantes atteintes d’une maladie rapidement 
