40 Transactions British Mycological Society. 
roots. The region of infection is thus perfectly continuous 
throughout the plant from the tip of the protocorm to the base 
of the inflorescence: as Bernard remarks, according to the evi- 
dence the whole of the mycelium harboured by a Neottia has 
for its single origin the mycelial filament which first penetrates 
the embryo*. 
The question arises as to whether root infection per se is 
obligate in orchids with abundant chlorophyll or whether it is 
a necessary evil. If the latter, one would expect the fungus to 
be lodged in the roots, though restricted in distribution. As 
stated above, all the cells entered seem to act as digestive cells 
in cultivated orchids. Is such digestion a device for protection 
or for nutrition? 
What has been happening to the fungus during these stages? 
The course of events was first followed by Bernard. As we have 
seen, the fungus enters at the suspensor end of the seed by the 
cells of the suspensor near the point of attachment (Odonto- 
glossum) or by the cells of the pole of the embryo where the 
suspensor is attached (Vanda). There appears to be an attrac- 
tion, though feeble, towards the place of entry. The first fila- 
ment entering the seed apparently excludes all others, though it 
may be of an attenuated form and unable itself to bring about 
germination. Bernard compared this with vaccination: the in- 
fection immunizes the seed. In successful germinations the 
fungus, after seed entry, follows the development of the cells 
forming mycelial balls in all the posterior portion of the seedling. 
According to Bernard, when the fungus reaches the cells bor- 
dering on the meristematic region digestion takes place. This 
is regarded as being analogous to phagocytosis such as occurs 
in animals where the white corpuscles of the blood attack, 
engulf and digest any invading micro-organisms: the cells in 
which the digestion takes place are the phagocytes. 
In general these may be regarded as definite cells often re- 
cognisable, even before infection on account of their nucleus 
sometimes becoming lobed. The balling of the fungus in the 
cells is compared with agglutination, and the mariner in which 
this occurs only in cells of the developing seedling which have 
achieved their growth is compared with cases of mortal infection 
where the balling is abandoned sooner or later and the fungus 
grows on in every direction and invades all the tissues indiffer- 
ently. 
* The association can be even more close under certain conditions. Flower 
scapes are frequently unable to pierce the humus covering them and the 
flowers and seeds develop underground, sometimes beneath the root-tufts 
which produce them. Mycelium apparently from the rhizome of the plant 
passes up the central cavity of the stem and infects the seeds in the sub- 
terranean fruits which are thus able to germinate. 
