232
F.
C. STEWARD AND Η. Y. MOHAN
RAM
IV. Morphogenetic Considerations
In Sections II and III above the problem of growth has been treated
as that of cells which grow in two phases, each distinctively controlled
by its own regulatory stimuli and mechanisms. Of necessity, however,
the cells have first been treated as units or entities. But in an article of
this kind it is of greater value if the ideas so derived may be applied also
to the problems which are presented by the organized development of
the plant body. This necessitates a reconsideration of the problems
presented by organized versus proliferative growth. Certain salient
morphological situations can be indicated which require, or lend themselves to, further examination from the points of view thus established.
This is the objective in this concluding section. Emphasis is, however,
given to ways of approach rather than to final solutions which have not
yet been realized.
A. Apomictic Development
Before commenting upon some problems concerning zygotic embryos,
one may refer to some instances in which embryos arise independently
of fertilization by the process known as apomictic development. Such
embryos may develop both from haploid and diploid embryo sacs. The
existence of these admittedly abnormal examples of development shows
that in the haploid embryo sacs, the egg or any other cell may produce
an embryo by the mere presence of a pollen tube in the embryo sac. In
adventive embryony, cells of the nucellus or the integument proliferate
and grow into the embryo sac, and develop into embryos. What is the
nature of the stimulus that triggers off this apomictic development?
In some rare cases, adventive embryos may develop independently of
pollination and fertilization, whereas in the majority of cases one or
both of these events have to take place (Maheshwari, 1950). Where
adventive embryony is autonomous, pollination and/or fertilization
accelerates the growth of the adventive embryos. Undoubtedly, in all
these cases some chemical stimulus of the general kind discussed in
Section II must be in operation. The success achieved by Fagerlind
(1946) in inducing adventive embryos in unpollinated ovaries by
application of auxin is indicative of the possible role of hormones in this
phenomenon. However, failures to promote the growth of such embryos
(van Overbeek et al., 1941; Chopra and Sachar, 1957) by the application
of growth substances furnish evidence to the contrary. Claims have been
made by Russian botanists (see Maheshwari and Ranga Swamy, 1958)
that nucellar cells may actually undergo somatic fusion with the male
nucleus of the pollen tube. Lacking the cytological evidence, the claims
F.
C. STEWARD AND Η. Y. MOHAN
RAM
IV. Morphogenetic Considerations
In Sections II and III above the problem of growth has been treated
as that of cells which grow in two phases, each distinctively controlled
by its own regulatory stimuli and mechanisms. Of necessity, however,
the cells have first been treated as units or entities. But in an article of
this kind it is of greater value if the ideas so derived may be applied also
to the problems which are presented by the organized development of
the plant body. This necessitates a reconsideration of the problems
presented by organized versus proliferative growth. Certain salient
morphological situations can be indicated which require, or lend themselves to, further examination from the points of view thus established.
This is the objective in this concluding section. Emphasis is, however,
given to ways of approach rather than to final solutions which have not
yet been realized.
A. Apomictic Development
Before commenting upon some problems concerning zygotic embryos,
one may refer to some instances in which embryos arise independently
of fertilization by the process known as apomictic development. Such
embryos may develop both from haploid and diploid embryo sacs. The
existence of these admittedly abnormal examples of development shows
that in the haploid embryo sacs, the egg or any other cell may produce
an embryo by the mere presence of a pollen tube in the embryo sac. In
adventive embryony, cells of the nucellus or the integument proliferate
and grow into the embryo sac, and develop into embryos. What is the
nature of the stimulus that triggers off this apomictic development?
In some rare cases, adventive embryos may develop independently of
pollination and fertilization, whereas in the majority of cases one or
both of these events have to take place (Maheshwari, 1950). Where
adventive embryony is autonomous, pollination and/or fertilization
accelerates the growth of the adventive embryos. Undoubtedly, in all
these cases some chemical stimulus of the general kind discussed in
Section II must be in operation. The success achieved by Fagerlind
(1946) in inducing adventive embryos in unpollinated ovaries by
application of auxin is indicative of the possible role of hormones in this
phenomenon. However, failures to promote the growth of such embryos
(van Overbeek et al., 1941; Chopra and Sachar, 1957) by the application
of growth substances furnish evidence to the contrary. Claims have been
made by Russian botanists (see Maheshwari and Ranga Swamy, 1958)
that nucellar cells may actually undergo somatic fusion with the male
nucleus of the pollen tube. Lacking the cytological evidence, the claims