Biology Reference
In-Depth Information
mimicry being enhanced by gene exchanges between crop and weed
races. In some cases, the mimicry is largely confined to vegetative similar-
ity, and the weedy plant simply produces and drops seed at or prior to
crop harvest. In other cases, mimicry extends to seed characteristics,
ensuring that weed seeds are harvested, and thus dispersed and planted,
with those of crop plants.The germination, growth, and reproduction of
the weed race typically match the phenology of the crop plant. These
adaptations have led to the introduction of many agroecotypes to new
continental areas as contaminants of grain used for seed.
Perhaps the most remarkable set of agroecotypes is that associated with
rice (Barrett 1983). Some of these are wild relatives and weedy races of
cultivated rice (
Oryza sativa
). Others are races of barnyard grass
(
Echinochloa crus-galli
) that have evolved to mimic the phenology, vegeta-
tive appearance, and seed morphology of cultivated rice. Two chromo-
some races of
E. crus-galli
—the tetraploid race,
phylopogon
, and the hexa-
ploid race,
oryzoides
—are worldwide in distribution in areas where
irrigated rice is grown.These races are essentially restricted to rice fields,
although other forms of
E. crus-galli
occur as weeds outside rice fields.
Needless to say, dispersal from their Asian region of origin has been as a
contaminant in rice seed.
Crop mimicry is perhaps most frequent among grain crops but is by
no means limited to grains (Barrett 1983). An agroecotype of vetch (
Vicia
sativa
), for example, possesses flattened seeds that mimic those of lentils
(
Lens esculenta
). False flax (
Camelina sativa
), a member of the mustard fam-
ily, is a mimic of cultivated flax (
Linum usitatissimum
).
Dispersal by Deliberate Introduction
Characteristics that make plants and animals valuable or attractive to
humans are, in a sense, preadaptations for their dispersal to new geograph-
ical regions. Crop, forestry, and horticultural plants, domestic animals and
exotic wildlife, aquacultural fish and shellfish, species prized by the pet
and aquarium trade—all have gained access to new geographical areas
because of their value or attractiveness to humans.
European colonization of many world regions led to the deliberate
introduction of many plants and animals. Many plants were introduced to
North America and other world regions from Europe in early Colonial
times because of their agricultural, medicinal, or herbal properties.
Numerous grasses and legumes were widely introduced, also beginning in
Colonial times, because of their real or supposed value as forages. Intro-
