Orchid Mycorrhiza. J. Ramsbottom. 31 
same orchid, but varies in different genera. It is only in the 
young root where root-hairs are present that the fungus is, as 
a rule, recognisable as such. The epidermal cells are not infected. 
The fungus usually enters the root through the root-hairs, but 
in some species it apparently is able to make use of any portion 
of the piliferous layer. The hyphae* pass through the external 
layers to a more or less definite zone, where they reach their 
maximum development, rapidly spreading and completely filling 
the cells. If an exodermis be present the hyphae pass through 
the thin-walled transfusion or passage cells. The first two or 
three cortical layers of the root are thus generally free from 
fungus except where the hyphae of infection pass through them: 
even in these there is no balling of the mycelium in the cells. 
In some genera (Habenaria) (Figs. 1 and 14), the fungal zone 
occupies roughly the third and fourth layer of cortical cells. In 
other genera (Neottia and Epipogum) the fungal zone of the root 
occupies three layers or so of cells separated from the endo- 
dermis by about half-a-dozen cell rows. In other cases practi- 
cally the whole of the cortex is occupied (Cymbidium and 
Odontoglossum). The central stele is never infected, the mycelium 
not entering the endodermis. The fungus also never infects the 
cells of the growing point of the root. Infected roots do not 
always show the endophyte in all their length, neither is it 
invariably present in a continuous zone. Infection does not 
generally occur once for all, but the hyphae from the soil infect 
the roots in several places and if the fungal zone be of several 
cells thickness it is frequently seen as patches in transverse 
section. Nor, asa rule, are all the roots of an orchid infected. 
Aerial roots particularly are free from fungus, the only excep- 
tions being where the roots are applied to the soil and are 
without chlorophyll. Such a case is shown in Fig. 15. Aerial 
roots can sometimes be found in such a position with the exposed 
portion green: in these circumstances if infection occur the 
fungus is restricted in distribution to the portion of the root 
without chlorophyll. In addition to cells containing chlorophyll 
those containing tannin, mucus, raphides and other crystals are 
never invaded by the fungus. Lateral roots are more frequently 
infected than main roots and in those genera with numerous 
roots (Orchis, Ophrys) according to Stahl only one out of three 
of the roots arising from the rhizome have fungus present in 
their cortex. Moreover, certain genera such as Listera and 
Epipactis, which have their chlorophyll particularly developed, 
seem to be irregularly infected, whereas plants poor in chloro- 
phyll, e.g. Limodorum and Corallorhiza, are well fungussed. All 
* Janse has shown that in Lecanorchis javanica the infecting hyphae are 
sometimes united into a mycelial ribbon. 
