242
F. C. STEWARD AND Η. Y. MOHAN RAM
basipetally and joined the vascular system below the region of the cuts,
the isolated apex grew enormously in length and formed a normal shoot.
The epidermis, cortex and vascular tissue were all differentiated from
the pith in basipetal sequence under the stimulus of the apical tissue.
Finally a wound periderm formed which then gave a rounded outline
to the stem. Thus the minimal activity of the apex proper was regulated
and maintained by substances or stimuli which emanated from the
subjacent pith tissue. The main stimulus to growth, whether qualitative
or quantitative, awaited the establishment of vascular connection
between the newly formed procambium and the original vascular tissue.
This experiment shows that the apex is an independent entity and also
that something more than the mere nutrients (water, salts, and sugars)
is furnished to the apex from the more mature tissue. In this context
one may refer also to the recovery of a normal shoot from crown gall
tumour cells which Braun (1959) achieved by grafting them on to normal
tobacco plants (see Sect. IV E).
The same general principle has also been used in the culture of single
plant cells. Muir et al. (1958) used a piece of nurse tissue in contact with
a filter paper on which a single cell had been placed to provide the
stimulus for division.
The basic causes as to why certain primordia develop into leaves and
others grow into buds are not yet understood. Surgical operations on the
primordia have revealed some basic facts. Depending on the angle and
depth of cuts, the primordia in some ferns may be either caused to grow
normally into leaves or modified to grow as buds (Wardlaw, 1949, 1950;
Wardlaw and Cutter, 1955; Cutter, 1956, 1957).
Similarly, if very young leaf primordia of the fern Osmunda cinnamomea (less than 1 mm) are isolated and cultured on a nutrient medium,
they first develop into buds and ultimately into whole plants (Steeves
and Sussex, 1957). Primordia larger than 1 mm could be grown into
leaves but although the leaves that were successfully cultured underwent
all the stages of vegetative growth, they always developed into sterile
fronds. Steeves and Sussex believe that the detached leaves lack the
factors responsible for cataphyll development, which in an attached
leaf would normally derive from the other parts of the plant body.
There is, therefore, a time factor which marks the degree of differentiation that controls the final fate of the primordium. The most striking
features of the artificially cultured leaves are their (1) smaller size, and
(2) fewer pinnae. The first results from a smaller number of cells, the
second is due to lack of meristematic activity along the margin of the
rachis.
F. C. STEWARD AND Η. Y. MOHAN RAM
basipetally and joined the vascular system below the region of the cuts,
the isolated apex grew enormously in length and formed a normal shoot.
The epidermis, cortex and vascular tissue were all differentiated from
the pith in basipetal sequence under the stimulus of the apical tissue.
Finally a wound periderm formed which then gave a rounded outline
to the stem. Thus the minimal activity of the apex proper was regulated
and maintained by substances or stimuli which emanated from the
subjacent pith tissue. The main stimulus to growth, whether qualitative
or quantitative, awaited the establishment of vascular connection
between the newly formed procambium and the original vascular tissue.
This experiment shows that the apex is an independent entity and also
that something more than the mere nutrients (water, salts, and sugars)
is furnished to the apex from the more mature tissue. In this context
one may refer also to the recovery of a normal shoot from crown gall
tumour cells which Braun (1959) achieved by grafting them on to normal
tobacco plants (see Sect. IV E).
The same general principle has also been used in the culture of single
plant cells. Muir et al. (1958) used a piece of nurse tissue in contact with
a filter paper on which a single cell had been placed to provide the
stimulus for division.
The basic causes as to why certain primordia develop into leaves and
others grow into buds are not yet understood. Surgical operations on the
primordia have revealed some basic facts. Depending on the angle and
depth of cuts, the primordia in some ferns may be either caused to grow
normally into leaves or modified to grow as buds (Wardlaw, 1949, 1950;
Wardlaw and Cutter, 1955; Cutter, 1956, 1957).
Similarly, if very young leaf primordia of the fern Osmunda cinnamomea (less than 1 mm) are isolated and cultured on a nutrient medium,
they first develop into buds and ultimately into whole plants (Steeves
and Sussex, 1957). Primordia larger than 1 mm could be grown into
leaves but although the leaves that were successfully cultured underwent
all the stages of vegetative growth, they always developed into sterile
fronds. Steeves and Sussex believe that the detached leaves lack the
factors responsible for cataphyll development, which in an attached
leaf would normally derive from the other parts of the plant body.
There is, therefore, a time factor which marks the degree of differentiation that controls the final fate of the primordium. The most striking
features of the artificially cultured leaves are their (1) smaller size, and
(2) fewer pinnae. The first results from a smaller number of cells, the
second is due to lack of meristematic activity along the margin of the
rachis.