Mycorrhiza — Arn old 
249 
and brown hyphae which nevertheless did not 
appear to lead to infection or any definite pat- 
tern of association. 
Radicles in many cases achieved a length of 
40 mm. without the appearance of branch roots. 
Up to this stage, no penetration of roots by 
hyphae was found though aggregates of my- 
celium formed in some cases a superficial collar- 
like layer anterior to the root-hair zone. 
Invasion of the radicle had not so far begun. 
After 6 or 7 weeks, usually at a time when 
leaves were expanding above the cotyledons, 
branch roots became obvious to the naked eye. 
A considerable proportion grew ahead free of 
infection, but others were attacked at the tips, 
as they emerged at the surface, by hyphae massed 
in a layer along the radicle. Microscopically the 
laterals so-enveloped by hyphae bore all the 
characteristics of mycorrhizas. As subsequent 
laterals reached the root exterior, some were 
transformed into mycorrhizas and others grew 
ahead, unimpeded, as nonmycorrhizal roots. 
In microtome sections of the radicle, no in- 
dication was found of intracellular penetration 
between mature epidermal cells. In emerging 
branch roots, intracellular penetration between 
epidermal cells was found only following radial 
elongation of the cells of the epidermis. This 
modification of the epidermal cells appears to be 
a necessary preliminary condition for penetra- 
tion by the mycorrhizal fungus. The prior ag- 
gregation of mycelium around the root-growing 
point to form a sheath appears to so alter the 
development of the epidermis as to enable later 
penetration. 
From the earliest stages of mycorrhiza forma- 
tion there was a tendency for the root apical 
meristem of thicker mycorrhizas to burst 
through the fungal mantle and to continue 
growth free of infection. Such recovery was not 
abundant at first but after 3 months became 
frequent. 
By mid-summer, mycorrhizas were present on 
most seedlings, though most radicles and a 
proportion of branch roots remained uninfected. 
MYCORRHIZAS OF TRANSPLANTED SEEDLINGS 
Transplanted mycorrhizal seedlings which 
were grown out-of-doors in pots in a friable 
mixture of oak-leaf mould, sand, fine gravel, and 
loam, continued to form mycorrhizas of a large 
"simple” type (Harley, 1937: 421-423) over a 
period of 3 years. 
With kitchen garden loams, results were 
variable. In heavier loams, large uninfected roots 
predominated. Mycorrhizas were abundant in 
lighter loams. 
In pits of pure river sand, about 2 ft. deep 
and 3 ft. wide, set in sandy soil, numerous fine 
and often distorted roots were formed, but 
mycorrhizas were rare. 
When mycorrhizal seedlings were trans- 
planted to pots containing vermiculite watered 
with a nutrient solution, no mycorrhizas were 
found after 6 months. The solution used was 
a basal medium devised by Melin and Nilsson 
(1950: 89) for mycorrhizal pine seedlings, 
with glucose and thiamine omitted. 
DISCUSSION 
It appears that the mycorrhizal association in 
N. solandri var. cliff ortioides reaches its highest 
intensity where humus is abundant. In some 
situations, such as boggy ground and clay, roots 
generally lack mycorrhizas, and in the case of 
transplanted seedlings mentioned above the as- 
sociation may be lost, indicating that the bal- 
ance of interacting growth between fungus and 
root is quite susceptible to variations in the 
edaphic conditions. 
When the circumstances are favourable, how- 
ever, it is apparent that massed colonization of 
the root surface of germinating seedlings is a 
necessary preliminary stage in the course of my- 
corrhizal infection. Solitary mycorrhizal hyphae 
appear not to be able to penetrate the root 
epidermis. 
The presence of sheets of mycelium along 
the radicle surface results in inhibition in growth 
of some emerging laterals, accompanied by a 
radial elongation of developing epidermal cells. 
Subsequent to this change in the epidermis, 
hyphae are able to effect intercellular penetra- 
tion, and a mycorrhiza results. It is possible 
that inhibition and modification of root growth 
is due to secretions of auxin by the mycorrhizal 
fungus (Slankis, 1950: 40-44). 
Lack of sufficient "inoculum potential” (Gar- 
