260
GEORGE G. LATIES
Recently, the work of Ordin et al. (1956) has done much to affirm the
view that the actual movement of water into the growing cell is entirely
osmotic. This supposition fits well with the notion that respiratory
energy is expended in growth before cell enlargement actually takes
place. In practice this distinction cannot frequently be made, since
water movement follows immediately upon the primary growth reactions.
Ordin and his co-workers have shown that Avena coleoptile sections
grow in solutions which are initially hypertonic only if an absorbable
solute is present in the external solution. The tissue is in diffusion pressure equilibrium with the external solution at all times, and growth in
hypertonic solution therefore depends upon the previous accumulation
of solute by the tissue. Osmotic pressure measurements upon the coleoptile cells bear out this interpretation. Even in hypotonic solution the
presence of an absorbable solute sustains initial growth rates which
would otherwise decline after several hours. Sucrose, which heretofore
has been considered to exert its pronounced effect on coleoptile section
growth by dint of its role as an energy source, may now be considered
to affect growth for osmotic reasons as well.
Since there is no evidence that auxin causes an increase in the concentration of osmotically active solutes within the cell (Hackett, 1952),
the diffusion pressure deficit (DPD) of growing cells must be increased
in the presence of auxin by a softening or plasticization of the cell walls.
Tagawa and Bonner (in press) have shown that such a plasticization does
in fact occur in Avena coleoptiles in response to auxin, while Cleland
and Bonner (1956) have demonstrated that auxin-induced metabolic reactions which result in wall softening occur in hypertonic solution under
conditions where growth cannot take place, and lead to growth in the
absence of auxin when the sections are subsequently transferred to
water. Thimann (1954) has performed similar experiments with pea
stem sections.
The concept of active water uptake, i.e., secretion of water into the cell
against an osmotic gradient (Bonner et al., 1953) has largely been abandoned. The experiments by Bogen (1953) purporting to demonstrate
active water uptake in the epidermal cells of the stem of Oenothera
franciscana have been reasonably explained by Ordin, Applewhite, and
Bonner in terms of osmotic water movement. Levitt (1948, 1953) has
long considered active water uptake to be unlikely on the basis of thermodynamic calculations of the quantity of energy necessary to retain water
within plant cells when osmotic equilibrium does not obtain. Levitt's
calculations presumed the ready permeability of plant cell protoplasts to
water. Ordin and Bonner (1956) have recently reaffirmed this presumption in full measure by experiments which measured the rate of move-
GEORGE G. LATIES
Recently, the work of Ordin et al. (1956) has done much to affirm the
view that the actual movement of water into the growing cell is entirely
osmotic. This supposition fits well with the notion that respiratory
energy is expended in growth before cell enlargement actually takes
place. In practice this distinction cannot frequently be made, since
water movement follows immediately upon the primary growth reactions.
Ordin and his co-workers have shown that Avena coleoptile sections
grow in solutions which are initially hypertonic only if an absorbable
solute is present in the external solution. The tissue is in diffusion pressure equilibrium with the external solution at all times, and growth in
hypertonic solution therefore depends upon the previous accumulation
of solute by the tissue. Osmotic pressure measurements upon the coleoptile cells bear out this interpretation. Even in hypotonic solution the
presence of an absorbable solute sustains initial growth rates which
would otherwise decline after several hours. Sucrose, which heretofore
has been considered to exert its pronounced effect on coleoptile section
growth by dint of its role as an energy source, may now be considered
to affect growth for osmotic reasons as well.
Since there is no evidence that auxin causes an increase in the concentration of osmotically active solutes within the cell (Hackett, 1952),
the diffusion pressure deficit (DPD) of growing cells must be increased
in the presence of auxin by a softening or plasticization of the cell walls.
Tagawa and Bonner (in press) have shown that such a plasticization does
in fact occur in Avena coleoptiles in response to auxin, while Cleland
and Bonner (1956) have demonstrated that auxin-induced metabolic reactions which result in wall softening occur in hypertonic solution under
conditions where growth cannot take place, and lead to growth in the
absence of auxin when the sections are subsequently transferred to
water. Thimann (1954) has performed similar experiments with pea
stem sections.
The concept of active water uptake, i.e., secretion of water into the cell
against an osmotic gradient (Bonner et al., 1953) has largely been abandoned. The experiments by Bogen (1953) purporting to demonstrate
active water uptake in the epidermal cells of the stem of Oenothera
franciscana have been reasonably explained by Ordin, Applewhite, and
Bonner in terms of osmotic water movement. Levitt (1948, 1953) has
long considered active water uptake to be unlikely on the basis of thermodynamic calculations of the quantity of energy necessary to retain water
within plant cells when osmotic equilibrium does not obtain. Levitt's
calculations presumed the ready permeability of plant cell protoplasts to
water. Ordin and Bonner (1956) have recently reaffirmed this presumption in full measure by experiments which measured the rate of move-