198
F.
C. STEWARD AND
Η. Y. MOHAN RAM
1. Growth Induction in Carrot Phloem Parenchyma
The secondary phloem of the carrot root normally forms a substantial
band of living tissue predominantly composed of parenchyma, which
would not normally grow again in the intact organ. The carrot root
cambium, however, does grow readily when it is isolated and subsequently cultured in a suitable nutrient medium; and, in fact, most early
carrot root tissue cultures (Gautheret, 1959) have had a cambial origin.
This is not surprising, since the maintained ability to divide is an inherent feature of the cambium. A major line of investigation stemmed
from the attempt to understand what factor, or factors, could restore
to the mature secondary phloem cells of the carrot root the maximum
ability for renewed and continued cell division. The trend of these
investigations was set in 1948, when Caplin and Steward tested various
substances and plant extracts for their growth-promoting ability and
discovered the very pronounced stimulus to the growth of small carrot
phloem explants by adding coconut milk to an otherwise complete
tissue culture medium. Explants of secondary phloem tissue, removed
aseptically from the carrot root from a point suitably distant from the
cambium, grow but little when placed upon a basal medium, such as
that of White (1943), which contains adequate salts, sugars, vitamins,
etc. In fact, very small explants isolated from most carrots merely
expand somewhat when treated in this way, due to the enlargement of
the preformed cells, but cell division is at a minimum. However, under
appropriate conditions and when 10% by volume (or even less) of whole
coconut milk is added to the medium, the explants embark upon a very
rapid and continued growth (Caplin and Steward, 1949). After a brief
lag period of about four days, growth continues exponentially for a
period, eventually slowing down to give a characteristic sigmoid timegrowth curve (Caplin and Steward, 1949). Such explants may increase
as much as 80 to 100 times their original fresh weight in about three
weeks, and about 25 to 28 thousand cells in the initial explant may
increase to between 2 and 3 million in approximately 21 days (Steward
and Shantz, 1956). The particular interest of this substantial stimulus
to growth in the otherwise mature and quiescent cells is that it is brought
about by the liquid endosperm of the coconut, which normally nourishes
the immature embryo.
2. Natural Sources of Cell Division Stimuli
One can, however, now generalize these observations as follows. In
comparable morphological situations there reside stimuli similar to those
contained in the coconut milk, for they will also induce rapid growth in
carrot and other tissue explants. Corn (Zea mays) endosperm furnishes
F.
C. STEWARD AND
Η. Y. MOHAN RAM
1. Growth Induction in Carrot Phloem Parenchyma
The secondary phloem of the carrot root normally forms a substantial
band of living tissue predominantly composed of parenchyma, which
would not normally grow again in the intact organ. The carrot root
cambium, however, does grow readily when it is isolated and subsequently cultured in a suitable nutrient medium; and, in fact, most early
carrot root tissue cultures (Gautheret, 1959) have had a cambial origin.
This is not surprising, since the maintained ability to divide is an inherent feature of the cambium. A major line of investigation stemmed
from the attempt to understand what factor, or factors, could restore
to the mature secondary phloem cells of the carrot root the maximum
ability for renewed and continued cell division. The trend of these
investigations was set in 1948, when Caplin and Steward tested various
substances and plant extracts for their growth-promoting ability and
discovered the very pronounced stimulus to the growth of small carrot
phloem explants by adding coconut milk to an otherwise complete
tissue culture medium. Explants of secondary phloem tissue, removed
aseptically from the carrot root from a point suitably distant from the
cambium, grow but little when placed upon a basal medium, such as
that of White (1943), which contains adequate salts, sugars, vitamins,
etc. In fact, very small explants isolated from most carrots merely
expand somewhat when treated in this way, due to the enlargement of
the preformed cells, but cell division is at a minimum. However, under
appropriate conditions and when 10% by volume (or even less) of whole
coconut milk is added to the medium, the explants embark upon a very
rapid and continued growth (Caplin and Steward, 1949). After a brief
lag period of about four days, growth continues exponentially for a
period, eventually slowing down to give a characteristic sigmoid timegrowth curve (Caplin and Steward, 1949). Such explants may increase
as much as 80 to 100 times their original fresh weight in about three
weeks, and about 25 to 28 thousand cells in the initial explant may
increase to between 2 and 3 million in approximately 21 days (Steward
and Shantz, 1956). The particular interest of this substantial stimulus
to growth in the otherwise mature and quiescent cells is that it is brought
about by the liquid endosperm of the coconut, which normally nourishes
the immature embryo.
2. Natural Sources of Cell Division Stimuli
One can, however, now generalize these observations as follows. In
comparable morphological situations there reside stimuli similar to those
contained in the coconut milk, for they will also induce rapid growth in
carrot and other tissue explants. Corn (Zea mays) endosperm furnishes