V.
DETERMINING FACTORS IN CELL GROWTH
195
restore certain otherwise adult living parenchyma cells to a state of
active cell division. The suggestion is that the growth and active cell
division so induced in the otherwise non-growing cells is evoked by
specific chemical substances which exert a regulatory effect upon
growth.
Thus the following general considerations emerge. Over and above the
ordinary nutrients and otherwise known hormones there are chemical
growth regulators which may intervene to re-establish growth in
otherwise non-growing cells, but in normal development these may also
control the pace and incidence of cell growth and division in selected
parts of the plant body. It is the bearing which these stimuli to cell
growth have upon the normal processes of morphogenesis that remain
to be considered in this chapter.
The classical approach to problems of tissue and organ culture, which
in the present century have owed so much to White and Gautheret, has
concentrated very largely upon the continued, potentially unlimited,
growth of tissue explants and upon the means to furnish a relatively
simple medium capable of maintaining growth without special regard
to its pace. Despite some dramatic successes (White, 1943, 1954;
Gautheret, 1942, 1959) the fact remains that by no means all organs and
tissues have responded to these treatments, and the successful cultures
have often grown rather slowly. Excised tissues from monocotyledonous
plants are outstanding examples which, for long, proved recalcitrant
and resisted many efforts to make them grow. Although some monocotyledonous tissue cultures have now been obtained by methods, yet
to be cited in this chapter, the fact remains that to this day even the
indefinitely continued aseptic culture of isolated root-tip tissue from
monocotyledonous plants is very difficult (Roberts and Street, 1955;
Almestrand, 1957) if, in fact, it has ever been satisfactorily achieved.
By contrast, many tissues and organs from dicotyledonous plants will
grow and multiply in aseptic cultures with comparative ease. The
obvious suggestion may be made that many of the earlier clones of
cultured dicotyledonous tissues were started originally from relatively
large explanted pieces of tissue which contained cambium. There is, in
fact, relatively little difficulty in the maintained activity of cambium,
for cambium is a special tissue in the dicotyledonous plant body which is
intrinsically endowed with the maintained ability to divide; albeit, this
ability is subject to seasonal control and cycles of dormancy and activity.
Thus, in this chapter, attention will be focused on the non-nutrient
factors that regulate the growth of cells. Attention will be directed first
to the factors which are involved in cell division, and subsequently to
factors that are involved predominantly in cell enlargement. This is a
customary, but somewhat arbitrary, division. Cells first multiply by
DETERMINING FACTORS IN CELL GROWTH
195
restore certain otherwise adult living parenchyma cells to a state of
active cell division. The suggestion is that the growth and active cell
division so induced in the otherwise non-growing cells is evoked by
specific chemical substances which exert a regulatory effect upon
growth.
Thus the following general considerations emerge. Over and above the
ordinary nutrients and otherwise known hormones there are chemical
growth regulators which may intervene to re-establish growth in
otherwise non-growing cells, but in normal development these may also
control the pace and incidence of cell growth and division in selected
parts of the plant body. It is the bearing which these stimuli to cell
growth have upon the normal processes of morphogenesis that remain
to be considered in this chapter.
The classical approach to problems of tissue and organ culture, which
in the present century have owed so much to White and Gautheret, has
concentrated very largely upon the continued, potentially unlimited,
growth of tissue explants and upon the means to furnish a relatively
simple medium capable of maintaining growth without special regard
to its pace. Despite some dramatic successes (White, 1943, 1954;
Gautheret, 1942, 1959) the fact remains that by no means all organs and
tissues have responded to these treatments, and the successful cultures
have often grown rather slowly. Excised tissues from monocotyledonous
plants are outstanding examples which, for long, proved recalcitrant
and resisted many efforts to make them grow. Although some monocotyledonous tissue cultures have now been obtained by methods, yet
to be cited in this chapter, the fact remains that to this day even the
indefinitely continued aseptic culture of isolated root-tip tissue from
monocotyledonous plants is very difficult (Roberts and Street, 1955;
Almestrand, 1957) if, in fact, it has ever been satisfactorily achieved.
By contrast, many tissues and organs from dicotyledonous plants will
grow and multiply in aseptic cultures with comparative ease. The
obvious suggestion may be made that many of the earlier clones of
cultured dicotyledonous tissues were started originally from relatively
large explanted pieces of tissue which contained cambium. There is, in
fact, relatively little difficulty in the maintained activity of cambium,
for cambium is a special tissue in the dicotyledonous plant body which is
intrinsically endowed with the maintained ability to divide; albeit, this
ability is subject to seasonal control and cycles of dormancy and activity.
Thus, in this chapter, attention will be focused on the non-nutrient
factors that regulate the growth of cells. Attention will be directed first
to the factors which are involved in cell division, and subsequently to
factors that are involved predominantly in cell enlargement. This is a
customary, but somewhat arbitrary, division. Cells first multiply by