96 
d 
Fig. 1 . a, An uninfected root, b, c, and d, mycor- 
rhizas. All life size. 
The rootcap of mycorrhizas is usually reduced 
to a single layer of cells confined on the outside 
by the fungal mantle which sheathes the whole 
apex and penetrates the later-formed epidermal 
cells. 
Differentiation of the epidermis of mycor- 
rhizas is initiated very much closer to the pro- 
meristem than is the case in uninfected roots, 
and is characterized by crowded divisions in the 
radial plane, the walls being as close together 
as 2.5 im in fine mycorrhizas, and slanting in a 
forward direction (Fig. 4). Clear vacuoles ap- 
pear in the developing epidermal cells but are 
later replaced by an opaque mass which fills the 
cells completely. This opaque mass absorbed 
all of the stains to which the sections were 
subjected. 
In large mycorrhizas the cortex may contain 
two or three layers of cells, but in finer my- 
corrhizas it may be reduced to a single layer 
or even replaced by the hypodermis (Fig. 5). 
Vacuolation of precursor cells of the cortex 
takes place earlier in mycorrhizas than in non- 
mycorrhizal roots. The vacuoles are at first clear, 
as is best shown by fixation in Marengo’s cyto- 
plasmic fixative (Marengo, 1952). Later the 
cortical cells of mycorrhizas become filled with 
an opaque homogeneous material which absorbs 
stains avidly. Millon’s test for protein in mature 
cortical cells was positive and strong. 
PACIFIC SCIENCE, Vol. XX, January 1966 
The staining reactions of the endodermis are 
similar to those of the cortex and epidermis but 
more intense. Vacuolation in the endodermis of 
mycorrhizas is precocious, as it is in the other 
tissues, and there is an early deposition of 
tannin in the endodermis. 
Maturation of stelar tissues occurs much closer 
to the promeristem in mycorrhizas than in 
mycorrhiza-free roots. 
All the foregoing histological characteristics 
of mycorrhizas of N. solandri var. cliffortioides 
are closely similar to those of mycorrhizas of 
F. sylvatica as reported by Clowes (1951), but 
one outstanding feature of Nothofagus mycor- 
rhizas which has not been reported for Fagus is 
the peculiar histogenetic pattern of the hypo- 
dermis. 
Vacuolation of the precursor cells of the hypo- 
dermis in mycorrhizas of Nothofagus takes place 
well in advance of vacuolation of the stelar cells. 
The first formed cells of the hypodermis are 
conspicuous extremely close to the promeristem. 
The cytoplasm shrinks against the walls to a 
lens-shaped blob in which the nucleus is em- 
bedded, and finally the entire protoplasmic 
content disappears. In longitudinal section an 
irregular line of clear cells can be seen leading 
back to a definite file of cells with thin buckled 
walls (Fig. 4). 
Despite the use of a wide range of fixatives 
and stains, the mature hypodermis appeared 
quite devoid of protoplasmic content. 
Hyphae were not found at any stage to enter 
this clear-layered hypodermis, which appears to 
be a barrier to further fungal invasion of the 
root. 
DISCUSSION 
Notwithstanding the undecided question of 
whether the meristematic layers which give rise 
to epidermis, cortex, stele, and rootcap in N. 
solandri var. cliffortioides are discrete histogens 
in the original sense of the term (Clowes, 1961), 
it is clear enough that in roots of Fagus and 
Nothofagus there exists the same fundamental 
type of organization of the apical meristem, 
which is consonant with the phylogenetic rela- 
tionship of the two genera. 
