83 
the Mycorrhiza of the Marattiaceae. 
All stages in the degeneration of the ‘ arbuscules ’ are irregularly 
distributed throughout the inner cortical cells of the host-root. In sections 
of infected roots, the cells which contain these masses of partially digested 
fungal mycelium can easily be distinguished, even with the naked eye, 
as pale yellowish strongly refractive spots, which stand out in sharp relief 
against the dull brown background of the other cells of the cortex. These 
clumps or masses of disorganized mycelium persist throughout the life 
of the host-root, yet the invaded cells show no obvious signs of injury, 
unless the complete disappearance of starch-grains from these cells can 
be interpreted as such. On the other hand, the host -cells not only keep the 
fungus in check, but presumably utilize as food part at least of the products 
of disorganization of the fungal hyphae. Moreover, the mycorrhizal cells 
exhibit no signs of approaching death until the root as a whole decays, their 
nuclei retaining a healthy appearance to the last (Text-fig. 4 , #, b ) . 
It is interesting to find that the mycelium is invariably absent from the 
tannin-cells of the host even when hyphae are abundant in adjacent cortical 
cells, and again, the fungal zone of the larger roots of Marattia Cooper i, 
Mre., is always situated outside the ring of mucilage-canals (PI. Ill, Fig. 7 ). 
Cook (18) has shown that tannin exerts a deleterious effect upon 
certain fungi, whilst Gallaud (he., p. 317 ) remarks of endotrophic fungi 
in general that they are repelled by secretory cells or by chlorenchyma. 
On the other hand, Peklo (35) found that the fungal symbiont of the mycor- 
rhiza of Carpinus and of Betida appears to show a preference for this 
substance, 
ii. Vesicles . 
The terminal portion of many of the hyphae swell up and give rise 
to structures which are generally known as vesicles (= ‘ vesicules ’ of Janse 
(25), N. Bernard (10), and others). These swellings, which are locally 
abundant and assume various shapes and dimensions, occupy either an 
inter- or an intracellular position amongst the cortical cells of the host-root 
(Text-fig. 6 , a , b, c ). They are never delimited by a septum from the sup- 
porting hyphae, from which they differ only in shape and size. When 
young they possess a thin hyaline membrane and contain a granular 
vacuolate cytoplasm in which numerous minute deeply staining bodies 
of irregular form can be distinguished (Text-fig. 6 , a , c ). 
As the vesicles increase in size, their wall irregularly thickens up and 
assumes a yellowish coloration exactly similar to that of the older hyphae, 
and, as in the latter, the contents of old vesicles tend to disappear. Numerous 
empty vesicles were observed in some of the roots (Text-fig. 6 , d). Now 
the fact that the vesicles rapidly lose their contents militates against the 
view that they are reproductive bodies of a conidial nature (cf. Campbell 
(13) ; also Noel Bernard ( 10 ), Fig. 9 , p. 249 ). Neither is there any reason 
for supposing that they are secretory products of the fungus, and they 
