H E T E R O B L A S T I C D E V E L O P M E N T I N P L A N T S
133
1. Primary Changes in Stem Thickness
The most consistent characteristic of the early development of the
stem is a progressive increase in diameter until a maximum thickness is
attained. There is frequently a subsequent reduction, especially in seed
plants, where it often indicates the onset of the flowering period. The
early obconical (i.e., inversely conical) development, referred to by
Troll and his associates as Erstarkungswachstum,
is readily seen in
pteridophytes and monocotyledons (Troll, 1937; Wetter and Wetter,
1954; Hagemann, 1964), but in dicotyledons is often masked by the
early inception of secondary thickening (Troll and Rauh, 1950).
Ά\\
ft (fil
y^ifJ
FIG. 2. Helianthus annuus. Longitudinal sections of shoot apices of plants of
different age. A and B, seedlings; C and D, older seedlings; E-G, transition to
floral apex. (Modified from Fig. 13 in Troll and Rauh, 1950.)
The gradual strengthening of the axis is clearly dependent on an enlargement of the apical regions (Fig. 2). Two processes are involved.
There is some enlargement of the free apex or promeristem itself, but
this is subordinate to a greater increase in the primary thickening of the
subapical region. Since the apex is the ultimate source of all tissues and
organs of the shoot, there have naturally been numerous studies of apical
activity and ontogeny, but a detailed account of these investigations is
not required in the present context. (For further details see Troll and
Rauh, 1950; Wetter and Wetter, 1954; Clowes, 1961; Allsopp, 1964a,
1965b; Wardlaw, 1965; Cutter, 1965; Nougarède, 1965; Prât, 1966.)
The enlargement of the shoot apex is sometimes accompanied by an increase in the number of leaf primordia aggregated in the apical bud. Such
133
1. Primary Changes in Stem Thickness
The most consistent characteristic of the early development of the
stem is a progressive increase in diameter until a maximum thickness is
attained. There is frequently a subsequent reduction, especially in seed
plants, where it often indicates the onset of the flowering period. The
early obconical (i.e., inversely conical) development, referred to by
Troll and his associates as Erstarkungswachstum,
is readily seen in
pteridophytes and monocotyledons (Troll, 1937; Wetter and Wetter,
1954; Hagemann, 1964), but in dicotyledons is often masked by the
early inception of secondary thickening (Troll and Rauh, 1950).
Ά\\
ft (fil
y^ifJ
FIG. 2. Helianthus annuus. Longitudinal sections of shoot apices of plants of
different age. A and B, seedlings; C and D, older seedlings; E-G, transition to
floral apex. (Modified from Fig. 13 in Troll and Rauh, 1950.)
The gradual strengthening of the axis is clearly dependent on an enlargement of the apical regions (Fig. 2). Two processes are involved.
There is some enlargement of the free apex or promeristem itself, but
this is subordinate to a greater increase in the primary thickening of the
subapical region. Since the apex is the ultimate source of all tissues and
organs of the shoot, there have naturally been numerous studies of apical
activity and ontogeny, but a detailed account of these investigations is
not required in the present context. (For further details see Troll and
Rauh, 1950; Wetter and Wetter, 1954; Clowes, 1961; Allsopp, 1964a,
1965b; Wardlaw, 1965; Cutter, 1965; Nougarède, 1965; Prât, 1966.)
The enlargement of the shoot apex is sometimes accompanied by an increase in the number of leaf primordia aggregated in the apical bud. Such