V.
DETERMINING
FACTORS IN CELL GROWTH
255
of this kind offers an ideal material for studying the origin of the crown
gall tumour cell and its capacity to recover.
Braun (1959) took pieces of the teratoma tissue, of the kind referred to
above, and cultured them in liquid containing White's basal medium
plus 10% extract of juice obtained from Vinca rosea crown gall tissue,
supplemented with 0-01% NAA. On shaking and within a short time, a
number of free cells and clumps of tissue were found suspended in the
liquid medium. Single cells were removed and cultured following the
procedure of Muir et al. (1958). In a few cases, small clumps of unorganized tissue were obtained from these single cells. The former were
transplanted on to stem apices of normal tobacco plants, from which
axillary buds had been removed. Any successful graft exhibited some
slight organization into shoots. The tips of shoots regenerated from such
grafts were severed and were quickly passed through a succession of
similar grafts onto normal tobacco stems. After such repeated graftings,
there was rapid growth leading to a complete recovery of the normal
habit (Plate 2, (2) for the type of grafts that are made). Explants from
the shoot of the completely recovered plant did not grow on a basal
medium that would support the crown gall tumour tissue; that is, the
recovery is not only morphological but is associated with a trend to
more restricted nutrition. Braun's (1959) experiment is unique, since it
demonstrates the totipotency of the crown gall cell, which is not only
able to produce cells of its own kind, but is also able to develop into a
normal shoot.
What then is the nature of transformation of a normal cell to a crown
gall tumour cell? Somatic mutations are not held to be responsible,
because eventually normal plants can be recovered from tumour cells.
Braun (1959) suggests that some autonomous or partially autonomous
entity which is subject to the effects of dilution in very rapidly dividing
cells (i.e. of the grafted tumour tissue) is responsible for the continuity
of the tumorous properties from one cell generation to the next. A
significant point, however, seems to be that here, as in the case of the
carrot, reorganization occurs after the tissue has been broken down to
the cellular level. The organizational stimulus of the host plant to the
grafted callus tissue may also be a factor in its recovery.
V. Summary
An attempt has been made above to bring many diverse observations
together under certain unifying ideas which are derived from a knowledge of the factors that control growth of plant cells by both division
and enlargement. Essentially these ideas are that once cells are endowed
by their origin, with that deep-seated or 'built-in' capacity to grow, the
DETERMINING
FACTORS IN CELL GROWTH
255
of this kind offers an ideal material for studying the origin of the crown
gall tumour cell and its capacity to recover.
Braun (1959) took pieces of the teratoma tissue, of the kind referred to
above, and cultured them in liquid containing White's basal medium
plus 10% extract of juice obtained from Vinca rosea crown gall tissue,
supplemented with 0-01% NAA. On shaking and within a short time, a
number of free cells and clumps of tissue were found suspended in the
liquid medium. Single cells were removed and cultured following the
procedure of Muir et al. (1958). In a few cases, small clumps of unorganized tissue were obtained from these single cells. The former were
transplanted on to stem apices of normal tobacco plants, from which
axillary buds had been removed. Any successful graft exhibited some
slight organization into shoots. The tips of shoots regenerated from such
grafts were severed and were quickly passed through a succession of
similar grafts onto normal tobacco stems. After such repeated graftings,
there was rapid growth leading to a complete recovery of the normal
habit (Plate 2, (2) for the type of grafts that are made). Explants from
the shoot of the completely recovered plant did not grow on a basal
medium that would support the crown gall tumour tissue; that is, the
recovery is not only morphological but is associated with a trend to
more restricted nutrition. Braun's (1959) experiment is unique, since it
demonstrates the totipotency of the crown gall cell, which is not only
able to produce cells of its own kind, but is also able to develop into a
normal shoot.
What then is the nature of transformation of a normal cell to a crown
gall tumour cell? Somatic mutations are not held to be responsible,
because eventually normal plants can be recovered from tumour cells.
Braun (1959) suggests that some autonomous or partially autonomous
entity which is subject to the effects of dilution in very rapidly dividing
cells (i.e. of the grafted tumour tissue) is responsible for the continuity
of the tumorous properties from one cell generation to the next. A
significant point, however, seems to be that here, as in the case of the
carrot, reorganization occurs after the tissue has been broken down to
the cellular level. The organizational stimulus of the host plant to the
grafted callus tissue may also be a factor in its recovery.
V. Summary
An attempt has been made above to bring many diverse observations
together under certain unifying ideas which are derived from a knowledge of the factors that control growth of plant cells by both division
and enlargement. Essentially these ideas are that once cells are endowed
by their origin, with that deep-seated or 'built-in' capacity to grow, the