38 i Flora Unveiled
PPNA. It is easy to imagine how the loss of seed dormancy might be selected for in
small gardens because the first gardeners would have preferentially harvested the rapidly
growing crops.
The second critical change that took place during cereal domestication was the devel-
opment of a tough rachis, the main stem or axis of the cereal head (ear). In wild cereals,
the rachis normally falls apart, or disarticulates, into dispersal units beginning at the top
of the inflorescence (Figure 3.4A,B). The rachis of a ripe head of wild wheat is thus easily
shattered into individual spikelets when touched or blown by the wind; hence, it is referred
to as a brittle rachis. A brittle rachis is an extremely undesirable trait from a human stand-
point because it drastically decreases the efficiency of harvesting. Plants with tough rachises
occasionally appear in wild cereal populations, but they are never selected for under natural
conditions because they prevent seed dispersal. During harvesting with sickles, however,
tough rachises would be artificially selected for by humans.
After cutting the tough- rachised domesticated varieties, the stalks can be transported
to another location for threshing. It is easy to distinguish the grains of wild and domestic
wheat varieties because the wild varieties have a smooth abscission scar where the spikelet
(a)Figure 3.4 Disarticulation of spikelet of wild versus domesticated wheat. A. Photograph of
ripe ear of wild wheat shedding a spikelet. B. Comparison of the abscission zones of wild versus
domesticated variety. Note the rough surface on the abscission zone of the domesticated wheat
variet y.
A is from Tenenbaum, David, http:// whyfiles.org/ shorties/ 199wheat/ ; B is from Smith, Bruce (1999), The
Emergence of Agriculture. Scientific American Library. W.H. Freeman & Co.