Jan. 1, 1925 
A Study of Variability in the Burt Oat 
19 
lemma may be extended into a swollen 
callosity, commonly called the callus. 
The dorsal surface of the lemma 
may be either hairy or glabrous, char¬ 
acters much used in separating oat 
species. Most wild species of oats 
are characterized by hairiness of the 
callus, lemma, and rachilla. The cal¬ 
lus, a somewhat swollen, thickened 
and hardened projection at the base 
of the lemma, often bears more or less 
conspicuous bristles, usually termed 
basal hairs. The presence of these 
hairs may be observed readily with¬ 
out magnification. 
The awn of oats is an extension of 
the midrib of the lemma, usually aris¬ 
ing at a point slightly above the mid¬ 
dle of the dorsal surface. In Avena 
fatua, A. sterilis , and various other 
wild species all lemmas are awned. 
In these the awns usually are stout 
and long and the basal portion is 
twisted dextrorsely or in a clockwise 
direction. That part of the awn 
above the twisted portion usually is 
bent or geniculate. In most culti¬ 
vated varieties derived from A. fatua 
and *4. sterilis the awn when present 
occurs only on the lower kernel of the 
spikelet and then it may be diminutive 
in size. In some cultivated varieties 
it occurs only rarely, even on the lower 
lemma. In varieties of A. byzantina 
awns occasionally occur on both ker¬ 
nels and sometimes they are twisted 
or geniculate. 
Trabut {135) observed a gradual 
reduction in the number and size of 
the awns in passing from the wild 
Avena sterilis to the cultivated strains 
represented by A. byzantina. Some 
authors believe the occurrence of 
numerous twisted and geniculate 
awns in cultivated oat varieties to be 
an indication of degeneracy resulting 
from an unfavorable environment. 
The palea or palet, the inner or upper 
bract or scale of the floret, is a thin 
parchmentlike scale, the margins . of 
which usually interlock with those of 
the lemma. The palet may be toothed 
at the apex. The tightness with which 
the lemma and palea clasp the caryop- 
sis is an important character in oat 
classification. In A. nuda they inclose 
the caryopsis rather loosely, while in 
most hulled varieties the caryopsis is 
firmly inclosed. 
The oat caryopsis is narrowly oblong 
or spindle shaped, deeply furrowed on 
the ventral surface, and usually cov¬ 
ered with fine hairs, especially at the 
upper end. The caryopsis usually has 
no characters of value for classification 
purposes. 
SPIKELET DISARTICULATION 
The separation of the lower floret of 
the oat spikelet from the rachilla or axis 
of the spikelet is here termed spikelet dis¬ 
articulation, in contrast to floret disjunc¬ 
tion or the separation of the florets of the 
spikelet from each other. Few experi¬ 
ments have been conducted on the his¬ 
tology of the oat spikelet, and as a result 
the exact structure of the rachilla is not 
well understood. 
In the wild species, Avena fatua and 
A. sterilis , and in most of the cultivated 
varieties the basal segment of the 
rachilla usually is short and thickened. 
Apparently, the basal segment of the 
rachilla and the projecting basal callus 
of the lower floret are united obliquely 
in the lateral plane, the callus being 
dorsal and the rachilla ventral. In 
the two wild species named, spikelet 
disarticulation takes place by means 
of an oblique abscission layer, appar¬ 
ently located in the cleavage plane be¬ 
tween the basal rachilla segment and 
the callus of the lower floret. It is 
possible that a true articulation be¬ 
tween the base of the lower lemma and 
the apex of its supporting rachilla seg¬ 
ment exists above this abscission layer, 
and that this abscission layer is formed 
in the tissue of the rachilla segment 
itself, but this is very improbable. 
Separation at this layer in that case 
would leave a portion of the basal 
rachilla segment attached to the lower 
floret. 
In the present paper it is assumed 
that the abscission layer is formed at 
the base of the callus of the lemma, and 
that below this abscission layer the 
tissue is rachilla, while above this layer 
it is lemma. In Avena sterilis , A. fatua, 
and some of their cultivated derivatives 
the separation at this definite oblique 
abscission layer leaves a well-defined 
deep oval cavity, commonly called the 
scar, or “ sucker-mouth,” in the face 
of the callus. A corresponding but 
shallower depression remains in the face 
of the disjoined basal segment of the 
rachilla. 
Zade {156) states that connection 
between these two parts, the callus 
and the rachilla segment, is only at 
the periphery in the wild species, while 
in the cultivated oat, A. sativa, the 
central portion also is solid, being filled 
with a parenchymatous tissue. The 
writers believe his observations prob¬ 
ably are correct for conditions at 
maturity, but that in fresh, immature 
plants the union of the rachilla and 
callus is solid both in wild species and 
their cultivated derivatives. The cav- 
