98 
Furthermore, it is evident that mycorrhizal 
infection does not alter the basic disposition of 
dermatogen, periblem, plerome, and calyptrogen 
in Nothofagus roots. The promeristem of mvcor- 
rhizas is reduced but not damaged or incapaci- 
tated. Not infrequently the apical meristem 
regains sufficient vigor to break through the 
fungal mantle and give rise to non-mycorrhizal 
roots, indicating a dynamic balance of growth of 
root and fungus. 
It has been suggested earlier (Arnold, 1959, 
I960) that the histological modifications found 
in Nothofagus mycorrhizas are consistent with 
the hypothesis that the mycorrhizal fungus ex- 
udes auxins or auxin-like substances which are 
a dominant factor in the morphogenesis and 
growth of Nothofagus roots. No doubt there 
are other accompanying effects of the presence 
of the fungal mantle over the tissues of the in- 
fected roots. 
In the very thorough study of mycorrhizas of 
Fagus sylvatica made by Clowes (1951), no 
mention is made of the structure and differentia- 
tion of the hypodermis. Morrison (1956), who 
FIG. 3. Longitudinal section of uninfected root, 
X 80. There is no premature differentiation of tissues 
as shown in Figure 4. 
PACIFIC SCIENCE, Vol. XX, January 1966 
studied the mycorrhizal condition in Nothofagus 
menziesii (Hook. f. ) Oerst., noted without fur- 
ther comment that in uninfected roots "the 
hypodermal layer consists of thin walled clear 
cells while the inner layer of the cortex consists 
of thick walled cells." No mention was made of 
the presence or otherwise of a hypodermis in 
mycorrhizas. 
In uninfected roots of Nothofagus solandri 
var. cliff ortioides a hypodermis of clear cells has 
been seen in transverse sections of some speci- 
mens but not in others, and it is presumed that 
Morrison’s observations were based on transverse 
sections probably somewhat remote from the 
apex. This presumption is made from observa- 
tions on longitudinal sections of uninfected 
roots of N. solandri var. cliffortioides in which 
no instances of a clear hypodermis were found 
even as far back as 1.5 cm from the promeri- 
stem. Vacuolation in the hypodermis of non- 
mycorrhizal roots of N. solandri var. cliffortioides 
is very much more gradual than in mycorrhizas. 
On the basis of the present observations on 
the histology and developmental pattern of the 
FIG. 4. Longitudinal section of a fine mycorrhiza, 
X 120. m, Mantle; e, epidermis; o, hypodermis; r, 
rootcap. 
