V.
DETERMINING FACTORS IN
CELL GROWTH
233
of nuclear fusion with a diploid vegetative cell are highly doubtful, but,
even so, contact with the pollen tube itself may furnish the necessary
stimulus.
Citrus presents a well known example in which several nucellar
embryos are formed within one seed. A recent study by Ranga Swamy
(1958); Maheshwari and Ranga Swamy (1958) shows that when segments
of nucelli from the micropylar part of the fertilized ovule are cultured
on a nutritive medium, supplemented with casein hydrolysate as a
source of nitrogen, these segments proliferate to produce pseudobulbils.
The latter have been maintained in subculture for over two and a half
years. These pseudobulbils give rise to plantlets with well differentiated
cotyledons, stem tip and root, when growth substances (e.g. gibberellic
acid or adenine) are added to the medium.
This work, therefore, demonstrates that nucellar cells which have
already received a stimulus through pollination of the ovary can be
successfully grown when isolated and cultured on a nutrient medium.
The inability to obtain a tissue culture of nucelli from unpollinated
ovaries, or from the chalazal part of the fertilized ovules, still needs a
chemical explanation. Although somatic in origin, the nucellar embryos
are distinct in the mode of their development from those of sporophytic
buds formed elsewhere on the plant, for whereas the latter grow into
seedlings directly, the former recapitulate the sequence of development of a seedling from a zygotic embryo and well defined cotyledons, radicle, plumule, epicotyl and hypocotyl are all distinctly seen
(Maheshwari, 1950). Challenging opportunities to describe the ultimate
stimuli in precise chemical terms, arise from the well documented cases
above, for the problem is clearly one of cell growth in a particular
biochemical setting.
B. Embryogenesis and Related Problems
As already intimated, the primary growing regions of the axis (shoot
and root) are already laid down in the embryo long before the seed
germinates. Indeed, in a few cases both foliar and floral primordia are
differentiated even in the dormant embryo, and the primordia of
adventitious seminal roots may also occur in many monocotyledonous
seeds. Thus, in large part, the rudiments of the plant body are already
laid down. Streptocarpus, a member of the Gesneriaceae, presents an
extreme case where the axis does not grow beyond the embryonic
(juvenile) stage, and the entire shoot is represented by the enlargement
of a single cotyledon (the other is suppressed) which may attain a size
of 22 cm long and 12 cm broad. Podostemon, Wolffia, etc. are other
examples of flowering plants in which the vascular tissue is virtually
DETERMINING FACTORS IN
CELL GROWTH
233
of nuclear fusion with a diploid vegetative cell are highly doubtful, but,
even so, contact with the pollen tube itself may furnish the necessary
stimulus.
Citrus presents a well known example in which several nucellar
embryos are formed within one seed. A recent study by Ranga Swamy
(1958); Maheshwari and Ranga Swamy (1958) shows that when segments
of nucelli from the micropylar part of the fertilized ovule are cultured
on a nutritive medium, supplemented with casein hydrolysate as a
source of nitrogen, these segments proliferate to produce pseudobulbils.
The latter have been maintained in subculture for over two and a half
years. These pseudobulbils give rise to plantlets with well differentiated
cotyledons, stem tip and root, when growth substances (e.g. gibberellic
acid or adenine) are added to the medium.
This work, therefore, demonstrates that nucellar cells which have
already received a stimulus through pollination of the ovary can be
successfully grown when isolated and cultured on a nutrient medium.
The inability to obtain a tissue culture of nucelli from unpollinated
ovaries, or from the chalazal part of the fertilized ovules, still needs a
chemical explanation. Although somatic in origin, the nucellar embryos
are distinct in the mode of their development from those of sporophytic
buds formed elsewhere on the plant, for whereas the latter grow into
seedlings directly, the former recapitulate the sequence of development of a seedling from a zygotic embryo and well defined cotyledons, radicle, plumule, epicotyl and hypocotyl are all distinctly seen
(Maheshwari, 1950). Challenging opportunities to describe the ultimate
stimuli in precise chemical terms, arise from the well documented cases
above, for the problem is clearly one of cell growth in a particular
biochemical setting.
B. Embryogenesis and Related Problems
As already intimated, the primary growing regions of the axis (shoot
and root) are already laid down in the embryo long before the seed
germinates. Indeed, in a few cases both foliar and floral primordia are
differentiated even in the dormant embryo, and the primordia of
adventitious seminal roots may also occur in many monocotyledonous
seeds. Thus, in large part, the rudiments of the plant body are already
laid down. Streptocarpus, a member of the Gesneriaceae, presents an
extreme case where the axis does not grow beyond the embryonic
(juvenile) stage, and the entire shoot is represented by the enlargement
of a single cotyledon (the other is suppressed) which may attain a size
of 22 cm long and 12 cm broad. Podostemon, Wolffia, etc. are other
examples of flowering plants in which the vascular tissue is virtually