74
JOHN G. TORREY
(1962) described a cell suspension culture derived from tissues of
Euphorbia in which he found isolated elements in the suspension which
were recognizable as mature differentiated tracheids. Whether these elements differentiated in isolation or were brought into suspension from
tissue clumps after their differentiation in a multicellular tissue was not
clear. Others (Muir et al, 1958; Gautheret, 1959) have illustrated differentiated vascular elements in liquid callus cultures. There lies in this
technique much promise for exploration of the problems dealing with the
chemical control of cell differentiation.
In Figs. 8-12 are illustrated, from different experimental situations,
tissues in which meristematic centers have differentiated from meristemoids to form specific cell types, either differentiated xylem elements or
starch storage cells. (See p. 70.)
There are a number of experimental situations in which vascular
tissues failed to develop in response to hormone treatment where they
might have been expected. These cases are of interest in terms of what
they might tell us of the conditions necessary for vascular tissue formations. Solberg and Higinbotham (1957) studied the effects of auxin and
sugars on differentiation in isolated pea epicotyl sections. They found
that IAA treatment with or without sucrose, although stimulating cell
elongations, resulted in decreased cell differentiation compared to untreated control segments. Segments still attached to the plant but with
the apex removed showed differentiation most similar to that of intact
plants. They concluded that auxin externally supplied to isolated segments in solution actually inhibited xylem maturation. No stimulation
of cell division occurred in these treatments. This result, which is in
direct contrast to that obtained by Sorokin et al. (1962), must be explicable in terms of differences of treatment or tissue susceptibility.
Jablonski and Skoog (1954) cultured tobacco pith sections on a nutrient
medium containing IAA, but found no vascular tissue differentiation.
Here again, at any of the concentrations of auxin tested, no cell divisions
were found. In our own studies with first-transfer pea root tips referred
to earlier, culturing the roots on top of an IAA-containing medium failed
to elicit vascular cambium initiation or any noticeable effect on xylem
differentiation. When the auxin was provided via the root base, both
cambial divisions and xylem formation occurred. As has been pointed
out by Wetmore and Rier (1963), vascular tissue formation in callus
tissue cultured on a nutrient medium seems to show completely random
distribution in the callus. Only when the auxin is applied at a "point
source" does one get any recognizable pattern of tissue formation.
In all these cases where hormones of the auxin type do not induce
vascular tissues, the common features seem to be the absence of a
JOHN G. TORREY
(1962) described a cell suspension culture derived from tissues of
Euphorbia in which he found isolated elements in the suspension which
were recognizable as mature differentiated tracheids. Whether these elements differentiated in isolation or were brought into suspension from
tissue clumps after their differentiation in a multicellular tissue was not
clear. Others (Muir et al, 1958; Gautheret, 1959) have illustrated differentiated vascular elements in liquid callus cultures. There lies in this
technique much promise for exploration of the problems dealing with the
chemical control of cell differentiation.
In Figs. 8-12 are illustrated, from different experimental situations,
tissues in which meristematic centers have differentiated from meristemoids to form specific cell types, either differentiated xylem elements or
starch storage cells. (See p. 70.)
There are a number of experimental situations in which vascular
tissues failed to develop in response to hormone treatment where they
might have been expected. These cases are of interest in terms of what
they might tell us of the conditions necessary for vascular tissue formations. Solberg and Higinbotham (1957) studied the effects of auxin and
sugars on differentiation in isolated pea epicotyl sections. They found
that IAA treatment with or without sucrose, although stimulating cell
elongations, resulted in decreased cell differentiation compared to untreated control segments. Segments still attached to the plant but with
the apex removed showed differentiation most similar to that of intact
plants. They concluded that auxin externally supplied to isolated segments in solution actually inhibited xylem maturation. No stimulation
of cell division occurred in these treatments. This result, which is in
direct contrast to that obtained by Sorokin et al. (1962), must be explicable in terms of differences of treatment or tissue susceptibility.
Jablonski and Skoog (1954) cultured tobacco pith sections on a nutrient
medium containing IAA, but found no vascular tissue differentiation.
Here again, at any of the concentrations of auxin tested, no cell divisions
were found. In our own studies with first-transfer pea root tips referred
to earlier, culturing the roots on top of an IAA-containing medium failed
to elicit vascular cambium initiation or any noticeable effect on xylem
differentiation. When the auxin was provided via the root base, both
cambial divisions and xylem formation occurred. As has been pointed
out by Wetmore and Rier (1963), vascular tissue formation in callus
tissue cultured on a nutrient medium seems to show completely random
distribution in the callus. Only when the auxin is applied at a "point
source" does one get any recognizable pattern of tissue formation.
In all these cases where hormones of the auxin type do not induce
vascular tissues, the common features seem to be the absence of a