240
F.
C. STEWARD AND Η. Y. MOHAN RAM
of which need not be discussed here. It is sufficient to state that there
exist in the shoot apex two distinct regions—the outer one or two layers
comprising the tunica (mantle layers of some authors) and the inner
core—the corpus. There is evidence for the belief that these layers are
periclinally arranged. The outer layer of tunica divides mostly anticlinally and contributes to the surface growth, while the increase in
volume is chiefly a function of the corpus.
A revolutionary concept has emerged from the works of such French
workers as Buvat (1952); according to this view, the group of cells
situated at the extreme tip of the shoot constitutes an inert meristem
('méristéme d'attente') (Fig. 5(a)). After a cytological investigation of
shoot apices in a few angiosperm species, Buvat (1952, 1953), Lance
(1952, 1953) and other French workers have come to the conclusion that
there is little mitotic activity in these extreme distal cells of the axis,
and that they contribute but little to the vegetative growth of the shoot.
It is only when a flower or an inflorescence is produced that these cells
regain their activity. The real shoot meristem, therefore (according to
these authors), is not terminal but subapical and lateral—in the form of
Tanneau initial' or 'initial ring'. It is from this ring that the leaf
primordia arise (see Fig. 5(a)).
Although severely criticized by anatomists and morphologists, the
'méristéme d'attente' concept has attracted the attention of physiologists. Physiological evidence that the extreme apical region in the root
is metabolically inactive (low oxygen uptake, reduced protein synthesis
and low enzyme activity) has already emerged from the works of Brown
and Broadbent (1950), Erickson and Goddard (1951), Brown, Reith and
Robinson (1952), Jensen (1955), etc. By the use of
14
C-labelled adenine,
Clowes (1956a, b; 1958) has successfully shown that, while there is
ready incorporation of the radioactively labelled purine into the DNA
of the meristematic cells in the subapical and peripheral part of the
root apex of Zea, Synapsis alba, Pistia, Eichornia, and other plants, the
central zone of apical cells does not synthesize DNA from externally
supplied
1 4
C adenine or phosphate. Thus, a quiescent central zone of
little mitotic activity is demonstrated by Clowes in his autoradiographs.
Jensen and Kavaljian (1958) and Hejnowicz (1959) have come to the
same conclusion, using a cytological technique. Further, there is
evidence for the belief that such quiescent zones also occur in shoots.
However, Clowes (1959a, b), Partanen and Gifford (1958), using the
same techniques of autoradiography have very recently failed to see any
quiescent zones in*the shoot apices. Although there are physiological
findings which are both for and against the 'méristéme d'attente'
concept which regards the distal cells of the shoot and root apices as
quiescent, one cannot dismiss their organizational role in the growth of
F.
C. STEWARD AND Η. Y. MOHAN RAM
of which need not be discussed here. It is sufficient to state that there
exist in the shoot apex two distinct regions—the outer one or two layers
comprising the tunica (mantle layers of some authors) and the inner
core—the corpus. There is evidence for the belief that these layers are
periclinally arranged. The outer layer of tunica divides mostly anticlinally and contributes to the surface growth, while the increase in
volume is chiefly a function of the corpus.
A revolutionary concept has emerged from the works of such French
workers as Buvat (1952); according to this view, the group of cells
situated at the extreme tip of the shoot constitutes an inert meristem
('méristéme d'attente') (Fig. 5(a)). After a cytological investigation of
shoot apices in a few angiosperm species, Buvat (1952, 1953), Lance
(1952, 1953) and other French workers have come to the conclusion that
there is little mitotic activity in these extreme distal cells of the axis,
and that they contribute but little to the vegetative growth of the shoot.
It is only when a flower or an inflorescence is produced that these cells
regain their activity. The real shoot meristem, therefore (according to
these authors), is not terminal but subapical and lateral—in the form of
Tanneau initial' or 'initial ring'. It is from this ring that the leaf
primordia arise (see Fig. 5(a)).
Although severely criticized by anatomists and morphologists, the
'méristéme d'attente' concept has attracted the attention of physiologists. Physiological evidence that the extreme apical region in the root
is metabolically inactive (low oxygen uptake, reduced protein synthesis
and low enzyme activity) has already emerged from the works of Brown
and Broadbent (1950), Erickson and Goddard (1951), Brown, Reith and
Robinson (1952), Jensen (1955), etc. By the use of
14
C-labelled adenine,
Clowes (1956a, b; 1958) has successfully shown that, while there is
ready incorporation of the radioactively labelled purine into the DNA
of the meristematic cells in the subapical and peripheral part of the
root apex of Zea, Synapsis alba, Pistia, Eichornia, and other plants, the
central zone of apical cells does not synthesize DNA from externally
supplied
1 4
C adenine or phosphate. Thus, a quiescent central zone of
little mitotic activity is demonstrated by Clowes in his autoradiographs.
Jensen and Kavaljian (1958) and Hejnowicz (1959) have come to the
same conclusion, using a cytological technique. Further, there is
evidence for the belief that such quiescent zones also occur in shoots.
However, Clowes (1959a, b), Partanen and Gifford (1958), using the
same techniques of autoradiography have very recently failed to see any
quiescent zones in*the shoot apices. Although there are physiological
findings which are both for and against the 'méristéme d'attente'
concept which regards the distal cells of the shoot and root apices as
quiescent, one cannot dismiss their organizational role in the growth of