230
F. C. STEWARD AND Η. Y. MOHAN
RAM
milk, 2,3,6-trichlorophenylacetic acid); by contrast, the post-climacteric
fruit does not respond to the same treatment (authors' unpublished
results).
In principle, therefore, there are here two similar phenomena. As
regulated by temperature during storage, the cells of the potato tuber
may retain, or lose, their ability for further growth and cell division.
In the banana, the stage of the cells in time with respect to the climacteric determines an essentially similar ability to respond to treatments
which activate cell division or expansion.
Thus the stimulus imparted to the otherwise mature cells of carrot,
by such factors as those in the coconut milk, represents the means of
reversing what would otherwise be a 'built-in trend toward senescence
and death', and once cells are thereby re-embarked upon growth, it
seems to proceed and to be potentially unlimited, at least in the presence
of coconut milk. Thus, in a pre-occupation with the factors that maintain
or cause a return to active growth, we may have failed to see the
significance of what may be learned from the progressive events
that lead to death. The significance of the climacteric in this regard
has yet to be elucidated, for it may represent a stage beyond which, in
terms of growth and synthesis, the cells have passed a point of no
return.
Lessons along these lines may be drawn from certain problems in
development. Whereas in some pericycles lateral roots originate freely,
opposite the protoxylem strands, in others (e.g. Narcissus, hyacinth)
this never occurs. The persistent ability of cells of some pericycles to
respond to stimuli, to divide and form roots, whereas others, which are
apparently similar, fail to do this suggests that the one type of cell
retains the basic ability to grow and divide far longer than the other.
Indeed it may be no accident that the stimuli to lateral root formation seem to emerge from the vicinity of the protoxylem (or lignifying elements of the stele), whereas it is around other lignifying
elements that orderly cell divisions occur in carrot tissue cultures and,
within the resultant cell layer, roots and eventually shoots may form
(Steward, Mapes and Mears, 1958).
In short, unspecialized cells may retain the prolonged ability for
growth, especially when they are appropriately stimulated by growthregulating substances. However, specialization of function (e.g. the
parenchyma of any leaves which are hard to grow in culture) often
curtails the duration and extent of this ability to grow. But even where
this property exists it may eventually encounter the onset of irreversible
change as at the climacteric (fruits), or the effect of external conditions
(e.g. temperature as in the case of the potato tuber).
Thus, curtailed longevity, earlier senescence and a decreased ability
F. C. STEWARD AND Η. Y. MOHAN
RAM
milk, 2,3,6-trichlorophenylacetic acid); by contrast, the post-climacteric
fruit does not respond to the same treatment (authors' unpublished
results).
In principle, therefore, there are here two similar phenomena. As
regulated by temperature during storage, the cells of the potato tuber
may retain, or lose, their ability for further growth and cell division.
In the banana, the stage of the cells in time with respect to the climacteric determines an essentially similar ability to respond to treatments
which activate cell division or expansion.
Thus the stimulus imparted to the otherwise mature cells of carrot,
by such factors as those in the coconut milk, represents the means of
reversing what would otherwise be a 'built-in trend toward senescence
and death', and once cells are thereby re-embarked upon growth, it
seems to proceed and to be potentially unlimited, at least in the presence
of coconut milk. Thus, in a pre-occupation with the factors that maintain
or cause a return to active growth, we may have failed to see the
significance of what may be learned from the progressive events
that lead to death. The significance of the climacteric in this regard
has yet to be elucidated, for it may represent a stage beyond which, in
terms of growth and synthesis, the cells have passed a point of no
return.
Lessons along these lines may be drawn from certain problems in
development. Whereas in some pericycles lateral roots originate freely,
opposite the protoxylem strands, in others (e.g. Narcissus, hyacinth)
this never occurs. The persistent ability of cells of some pericycles to
respond to stimuli, to divide and form roots, whereas others, which are
apparently similar, fail to do this suggests that the one type of cell
retains the basic ability to grow and divide far longer than the other.
Indeed it may be no accident that the stimuli to lateral root formation seem to emerge from the vicinity of the protoxylem (or lignifying elements of the stele), whereas it is around other lignifying
elements that orderly cell divisions occur in carrot tissue cultures and,
within the resultant cell layer, roots and eventually shoots may form
(Steward, Mapes and Mears, 1958).
In short, unspecialized cells may retain the prolonged ability for
growth, especially when they are appropriately stimulated by growthregulating substances. However, specialization of function (e.g. the
parenchyma of any leaves which are hard to grow in culture) often
curtails the duration and extent of this ability to grow. But even where
this property exists it may eventually encounter the onset of irreversible
change as at the climacteric (fruits), or the effect of external conditions
(e.g. temperature as in the case of the potato tuber).
Thus, curtailed longevity, earlier senescence and a decreased ability