V. DETERMINING
FACTORS IN
CELL GROWTH
241
the vegetative plant body (Wardlaw, 1957a, b). Thus, one may visualize the possibility that these so-called quiescent regions may be active
centres for certain types of syntheses and that they may transmit chemical stimuli that control growth by cell division in the surrounding
meristematic cells and by expansion elsewhere in the apex.
Gifford (1954) and Wardlaw (1957a) offer well known anatomical and
morphological findings to disprove the French concept. Anatomists
have recognized these relatively quiescent zones ['Mother cell zone' of
Popham (1951)] at least in the apices of a few vascular plants and the
occurrence of active mitotic divisions in the apical cells of pteridophytes
(Lycopodium, Psilotum, Dryopteris, Equisetum) Fig. 5(6). Knowledge of
gymnosperms (Pinns, Ginkgo) and many angiosperms does not lend
support to the fact that these large cells are necessarily inert. Dichotomous branching of stem apices, both induced and natural, the induction
of periclinal chimeras (Dermen, 1941, 1947, 1953; Satina, Blakeslee and
Avery, 1940; Satina and Blakeslee, 1941, 1943) are further examples
which show that the apical region does essentially contribute to the
vegetative growth of the plant body.
While very small root tips, which do not contain any mature tissues,
could be successfully grown to maturity with special growth supplements
in the nutritive medium, little is known about the requirements of the
extremely short shoot apices.
In this connection it is instructive to recall the experiments of Ball
(1948) which he designed to elucidate some of the basic problems
associated with the shoot apex. By two longitudinal cuts at right angles
to one another, passing through the centre, Ball split a lupine shoot
apex into four parts (the basal part of the shoot, however, was intact).
Thus the apical group of initials was disorganized. Interestingly enough,
each part behaved as a new meristem and produced leaf primordia not
only on the uninjured side but toward the cut surface also. The four
parts developed as independent shoots. This not only demonstrates the
potentiality of a very few apical cells to regenerate the whole meristem
but also the role of the subjacent tissues in supporting such recovery by
supplying the necessary stimuli.
In another experiment, Ball (1952) isolated the central apical dome
of the growing region by four longitudinal cuts on the surface in such a
way that all connections with the leaf primordia and procambium were
severed. Thus the distal group of meristematic cells rested upon a
four-sided column, squarish in cross section, of pith cells, which was the
sole connection with the rest of the plant body. The subjacent column
of pith permitted a slow growth and formation of one or two rudimentary
primordia at the apex in the vicinity of the cuts, concomitantly with
the formation of a ring of procambium. Once the procambium developed
FACTORS IN
CELL GROWTH
241
the vegetative plant body (Wardlaw, 1957a, b). Thus, one may visualize the possibility that these so-called quiescent regions may be active
centres for certain types of syntheses and that they may transmit chemical stimuli that control growth by cell division in the surrounding
meristematic cells and by expansion elsewhere in the apex.
Gifford (1954) and Wardlaw (1957a) offer well known anatomical and
morphological findings to disprove the French concept. Anatomists
have recognized these relatively quiescent zones ['Mother cell zone' of
Popham (1951)] at least in the apices of a few vascular plants and the
occurrence of active mitotic divisions in the apical cells of pteridophytes
(Lycopodium, Psilotum, Dryopteris, Equisetum) Fig. 5(6). Knowledge of
gymnosperms (Pinns, Ginkgo) and many angiosperms does not lend
support to the fact that these large cells are necessarily inert. Dichotomous branching of stem apices, both induced and natural, the induction
of periclinal chimeras (Dermen, 1941, 1947, 1953; Satina, Blakeslee and
Avery, 1940; Satina and Blakeslee, 1941, 1943) are further examples
which show that the apical region does essentially contribute to the
vegetative growth of the plant body.
While very small root tips, which do not contain any mature tissues,
could be successfully grown to maturity with special growth supplements
in the nutritive medium, little is known about the requirements of the
extremely short shoot apices.
In this connection it is instructive to recall the experiments of Ball
(1948) which he designed to elucidate some of the basic problems
associated with the shoot apex. By two longitudinal cuts at right angles
to one another, passing through the centre, Ball split a lupine shoot
apex into four parts (the basal part of the shoot, however, was intact).
Thus the apical group of initials was disorganized. Interestingly enough,
each part behaved as a new meristem and produced leaf primordia not
only on the uninjured side but toward the cut surface also. The four
parts developed as independent shoots. This not only demonstrates the
potentiality of a very few apical cells to regenerate the whole meristem
but also the role of the subjacent tissues in supporting such recovery by
supplying the necessary stimuli.
In another experiment, Ball (1952) isolated the central apical dome
of the growing region by four longitudinal cuts on the surface in such a
way that all connections with the leaf primordia and procambium were
severed. Thus the distal group of meristematic cells rested upon a
four-sided column, squarish in cross section, of pith cells, which was the
sole connection with the rest of the plant body. The subjacent column
of pith permitted a slow growth and formation of one or two rudimentary
primordia at the apex in the vicinity of the cuts, concomitantly with
the formation of a ring of procambium. Once the procambium developed