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F. C. STEWARD AND Η. Y. MOHAN RAM
division and then they enlarge, but the two processes merge one into the
other. Continued growth involves both processes, which must normally
proceed sequentially and in balance. Some cells which have ceased to
divide may differentiate by enlargement and fulfil their role in the plant
body as living parenchyma, and it is especially in this area that the role
of the so-called auxins has been stressed. It is in very recent years that
knowledge has been gained about the multiplicity of factors that
stimulate or regulate cell division, and Miller et al. (1956) have coined
the term 'kinins' to designate substances which fulfil this role.
II. Phases and Factors in Cell Growth
A. Stimuli to Cell Division in Plants
Little is known about the chemical nature of those natural stimuli to
cell division which operate in the most dramatic situations as they occur
in the plant body. Pollination without the act of fertilization itself may,
in some cases, exert a stimulus to cell division, even inducing the
development of the ovary. Dead pollen and even spores of Lycopodium
(Gärtner, 1849) have been known to stimulate the growth of ovary walls
even in situations quite foreign to those in which they normally act.
But the great stimulus to cell division begins with the act of fertilization
conveying to the zygote, what Sinnott aptly called, its 'built-in capacity
to grow', and, as noted above, triple fusion conveys another growth
stimulus which leads to the formation of the endosperm.
Although much may be said about these acts of nuclear fusion and
the imprinting upon the DNA of the nucleus of that coded information
which is believed to control later development, the real reason why the
resultant cell may divide and grow, as it incorporates nutrients into a
replicate of itself, is still a baffling problem. In other words, the coded
information stored in the DNA of the fusion nucleus may direct the
course of growth but does not necessarily furnish its drive or 'thrust'.
After the initial stimulus and the formation of an organized embryo,
many cells which remain alive in the plant body fulfil their ultimate
functions as parenchyma in one tissue or another. Some of these cells
are destined to lose, apparently irreversibly, their ability for further
division, and this occurs early in their ontogeny. This is true of such
parenchyma as those normally found in pome fruits, in the tissues of
many monocotyledons, as for example the parenchyma of the bulb
scale of the onion, to cite a few of many examples. In dicotyledonous
plants, however, and particularly in the case of tissues that have a
secondary origin, the living cells of many storage organs, or of permanent
tissues of the plant body, may be restored to the actively growing state
by various means. The stimuli, chemical or otherwise, which cause this
F. C. STEWARD AND Η. Y. MOHAN RAM
division and then they enlarge, but the two processes merge one into the
other. Continued growth involves both processes, which must normally
proceed sequentially and in balance. Some cells which have ceased to
divide may differentiate by enlargement and fulfil their role in the plant
body as living parenchyma, and it is especially in this area that the role
of the so-called auxins has been stressed. It is in very recent years that
knowledge has been gained about the multiplicity of factors that
stimulate or regulate cell division, and Miller et al. (1956) have coined
the term 'kinins' to designate substances which fulfil this role.
II. Phases and Factors in Cell Growth
A. Stimuli to Cell Division in Plants
Little is known about the chemical nature of those natural stimuli to
cell division which operate in the most dramatic situations as they occur
in the plant body. Pollination without the act of fertilization itself may,
in some cases, exert a stimulus to cell division, even inducing the
development of the ovary. Dead pollen and even spores of Lycopodium
(Gärtner, 1849) have been known to stimulate the growth of ovary walls
even in situations quite foreign to those in which they normally act.
But the great stimulus to cell division begins with the act of fertilization
conveying to the zygote, what Sinnott aptly called, its 'built-in capacity
to grow', and, as noted above, triple fusion conveys another growth
stimulus which leads to the formation of the endosperm.
Although much may be said about these acts of nuclear fusion and
the imprinting upon the DNA of the nucleus of that coded information
which is believed to control later development, the real reason why the
resultant cell may divide and grow, as it incorporates nutrients into a
replicate of itself, is still a baffling problem. In other words, the coded
information stored in the DNA of the fusion nucleus may direct the
course of growth but does not necessarily furnish its drive or 'thrust'.
After the initial stimulus and the formation of an organized embryo,
many cells which remain alive in the plant body fulfil their ultimate
functions as parenchyma in one tissue or another. Some of these cells
are destined to lose, apparently irreversibly, their ability for further
division, and this occurs early in their ontogeny. This is true of such
parenchyma as those normally found in pome fruits, in the tissues of
many monocotyledons, as for example the parenchyma of the bulb
scale of the onion, to cite a few of many examples. In dicotyledonous
plants, however, and particularly in the case of tissues that have a
secondary origin, the living cells of many storage organs, or of permanent
tissues of the plant body, may be restored to the actively growing state
by various means. The stimuli, chemical or otherwise, which cause this