44 Transactions British Mycological Soctety. 
mycelium with a comparatively thick wall. The middle layer 
is composed of a bundle of large thin-walled mycelia with 
numerous septa. The inner cavity of the strand is traversed by 
a loose bundle of very fine longitudinal hyphae rich in proto- 
plasmic contents. 
When the tuber of Gastrodia is attacked by the rhizomorph, , 
infection is effected by a sucker-like branch of the strand which 
penetrates the cortical cell layers, partly compressing the under- 
lying cells and partly dissolving their walls. This mode of in- 
fection is, of course, quite different from the ordinary endophytic 
mycorrhizal type where infection is effected as a rule by a single 
hypha (cf. p. 31). It very much resembles the manner in which 
the parasitic Cuscuta attacks its hosts, the rhizomorph creeping 
over the surface of the tuber and giving off the infection branches 
at intervals. On entering the tuber the hyphae of the various 
portions of the strand essentially retain their structure. The 
infected area of the tuber may be divided into three regions, 
according to the structure of the cells and the nature of the 
hyphae contained within them. The external region is composed 
of two or three layers of cells which contain a densely entangled 
mass of comparatively thick-walled hyphae; the middle region 
is similarly composed, except that the hyphae are generally 
thin-walled and of various breadths and often arranged as a 
pseudoparenchyma; the innermost region has large cells each 
containing a few, slender, slightly curved hyphae. The three 
regions correspond to the zones in the rhizomorph. The hyphae 
of each region show characteristic alterations. They are per- 
manent in the first region; in the second they undergo self- 
disorganization ; while in the third they are mostly consumed by 
the cells of the host.. The mode of development of the fungus 
in the middle region simulates the ordinary clumping seen in 
most orchids, but the course of events is different in that the 
protoplast is consumed by the hyphae before their collapse takes 
place. The destruction of the protoplast shows the parasitic 
properties of the hyphae. The cells of the inner regions are 
apparently metabolic centres of the orchid where the food 
materials are elaborated. The nucleus and cytoplasm undergo 
remarkable alterations, and secondary products appear indi- 
cating considerable activities. After the disappearance of the 
hyphae the nucleus resumes its original form and structure, 
while the cytoplasm again becomes fibrous and vacuolate. Starch 
grains disappear from all the mycorrhizal cells, to reappear in 
the inner region with the cessation of metabolic activity. 
The association of tuber and rhizomorph takes place quite 
occasionally. If a tuber forms mycorrhiza it can give rise to a 
full grown off-set which remains dormant during the winter and 
