88
DAVID D. KECK
environment, inasmuch as they are held apart by very strong genetic
barriers.
7. The genetic complexes of species may group into even more comprehensive cytological complexes based on chromosome homology. These
cytological units are probably of deep evolutionary significance. They
may parallel the cenospecies or genetic species complexes but are not
necessarily identical with them.
8. Classification should be based on a consideration of all the evidence—morphological, ecological, cytological, and genetical—because
each line contributes to a better understanding of the biological picture.
Such a classification is more highly objective, and hence more dependable and satisfactory than the older classifications.
The successful artificial production of a new species of Layia through
amphiploidy has disclosed an evolutionary pattern that has not been
exploited to any extent in the genus. Layia pentachaeta,
η = 8, crossed
with L. platyglossa, η — 7, gave rise to hybrid plants that produced
diploid sex cells; in the F 2 generation these yielded 3 relatively sterile
amphiploid individuals, which in turn produced a relatively uniform F 3 ,
L. pentaglossa, η = 15 (Clausen et al 1945). Layia panictdata possibly
arose in this manner. Because amphiploids are capable of displaying
appreciable heterosis (hybrid vigor), it is possible that amphiploidy
may play a greater role in future speciation in Layia.
XVI. APOMIXIS
Apomixis is an asexual method of propagation that is rather common
among seed plants, although fortunately for the taxonomist it is much
less common than sexual reproduction. The number of studies made on
apomixis in higher plants is very large, and because the mechanism and
the causes of the process are varied, it is not surprising that a complex
and often conflicting terminology developed to explain the phenomena.
A very comprehensive treatment of the entire subject by Gustafsson
(1946, 1947a, b) is recommended to the reader who wishes to go deeper
into these aspects. Extensive lists of examples can also be found in
Stebbins (1941) and Nygren (1954).
In some genera, such as Poa, a species may be partially apomictic,
with part of the offspring produced through regular sexual processes and
part asexually through apomixis. As apomixis is merely a specialized
method of vegetative propagation, it follows that the offspring of an
apomictic plant will all be alike and like the parent. A species such as
Poa nervosa (Hook.) Vasey, which has a wide distribution in the western
United States and is apomictic over most of its range, is highly uniform.
DAVID D. KECK
environment, inasmuch as they are held apart by very strong genetic
barriers.
7. The genetic complexes of species may group into even more comprehensive cytological complexes based on chromosome homology. These
cytological units are probably of deep evolutionary significance. They
may parallel the cenospecies or genetic species complexes but are not
necessarily identical with them.
8. Classification should be based on a consideration of all the evidence—morphological, ecological, cytological, and genetical—because
each line contributes to a better understanding of the biological picture.
Such a classification is more highly objective, and hence more dependable and satisfactory than the older classifications.
The successful artificial production of a new species of Layia through
amphiploidy has disclosed an evolutionary pattern that has not been
exploited to any extent in the genus. Layia pentachaeta,
η = 8, crossed
with L. platyglossa, η — 7, gave rise to hybrid plants that produced
diploid sex cells; in the F 2 generation these yielded 3 relatively sterile
amphiploid individuals, which in turn produced a relatively uniform F 3 ,
L. pentaglossa, η = 15 (Clausen et al 1945). Layia panictdata possibly
arose in this manner. Because amphiploids are capable of displaying
appreciable heterosis (hybrid vigor), it is possible that amphiploidy
may play a greater role in future speciation in Layia.
XVI. APOMIXIS
Apomixis is an asexual method of propagation that is rather common
among seed plants, although fortunately for the taxonomist it is much
less common than sexual reproduction. The number of studies made on
apomixis in higher plants is very large, and because the mechanism and
the causes of the process are varied, it is not surprising that a complex
and often conflicting terminology developed to explain the phenomena.
A very comprehensive treatment of the entire subject by Gustafsson
(1946, 1947a, b) is recommended to the reader who wishes to go deeper
into these aspects. Extensive lists of examples can also be found in
Stebbins (1941) and Nygren (1954).
In some genera, such as Poa, a species may be partially apomictic,
with part of the offspring produced through regular sexual processes and
part asexually through apomixis. As apomixis is merely a specialized
method of vegetative propagation, it follows that the offspring of an
apomictic plant will all be alike and like the parent. A species such as
Poa nervosa (Hook.) Vasey, which has a wide distribution in the western
United States and is apomictic over most of its range, is highly uniform.