If seed coat removal does not allow germination under favorable conditions, control obviously lies
within the embryo (exalbuminous seeds) and/or endosperm (albuminous seeds). Albuminous seeds are
composed primarily of endosperm; the embryo is relatively small. Warm, moist conditions for a period
of 2 to 4 months following seed dispersal are usually required for coconut and ginkgo embryos to enlarge
to the point where they are capable of germination. Some species [e.g., cowparsnip (Heracleum sphon-
dyliumL.)] require chilling for embryo development [45]; embryos develop very slowly at 15°C.
In exalbuminous seeds, the embryo is fully developed at maturity. However, many such embryos will
not germinate, or germinate only sluggishly, when the seed coat is removed. Exposure to moisture and
low temperatures (0 to 10°C) for periods of 1 to 20 weeks (cool, moist stratification) is often required to
permit germination. Little or no growth of the embryo occurs during this time; the treatment alters the em-
bryo’s metabolism without affecting its morphology.
F. Epicotyl Dormancy
Some seeds [e.g., tree peony (Paeonia suffruticosaHaw.)] germinate readily without special treatment,
but the epicotyl (shoot) will not elongate unless chilled [46]. Chilling prior to germination is ineffective.
G. Double Dormancy
More than one mechanism may prevent the germination of a seed. Certain legumes [e.g., redbud (Cercis
canadensis)] not only have hard seed coats but their embryos must be chilled before germination can oc-
cur (Table 3). Scarification, followed by moist chilling, breaks their dormancy. In other seeds (e.g., Tril-
lium erectum) the radicle and the epicotyl both require chilling, but the periods at low temperature must
be sequential. The first period permits radicle protrusion, the second shoot emergence [47].
H. Thermodormancy and Secondary Dormancy
All of the types of dormancy just described are examples of primary dormancy, in which germination is
prevented by conditions within the seed at the time it matures on the plant. Thermodormancycan be in-
duced by exposure of seeds that are capable of germination at low temperatures (10 to 15°C) to high tem-
peratures (25 to 30°C). This can occur in lettuce, for example, when soil temperatures are very high. Sec-
ondary dormancyis induced when a seed that is not dormant when shed, or whose dormancy has been
partially broken, is exposed to unfavorable conditions, such as high temperature or drying. In seeds that
are chilled for less than the required time, for example, premature exposure to high temperature can elim-
inate the effects of prior chilling.
IV. BUD DORMANCY
Following bud break in the spring, shoot growth is relatively slow at the beginning of the season, accel-
erates with time, then slows and eventually stops. This pattern tends to occur even at constant tempera-
ture. As noted before, growth tends to be cyclical. Even in the humid tropics flushes of growth occur in a
more or less random fashion; one shoot on a tree may be growing rapidly while growth of another is neg-
ligible or nil. In contrast, growth of perennials in the temperate zone is synchronized. Growth ceases in
mid- to late summer and the plants pass through a dormant period lasting for several months.
Fuchigami et al. [48] have described this pattern of growth as a sine wave (degree growth stage
model), with 0° representing the end of ecodormancy/beginning of active growth; 90°, the end of active
growth (maturity induction point beginning of paradormancy); 180°, “vegetative maturity” (beginning
of endodormancy); 270°, the time of deepest endodormancy; and 315°, the end of endodormancy/begin-
ning of ecodormancy (Figure 5). Note that phase transition is gradual rather than abrupt; endodormancy
does not end one day and ecodormancy begin the next; rather, there is a gradual transition from one phase
to the next. During the early part of the summer, removal of the shoot apex and/or defoliation relieves api-
cal dominance and permits growth of the lateral buds. This is true not only in woody plants but in many
herbaceous ones as well. Horticulturists remove the apical portion (“pinch”) chrysanthemums and petu-
nias to force branching and thereby create more attractive plants. Arboriculturists use the same practice
to stimulate the formation of lateral branches. At this time, the axillary buds are paradormant(see ear-
168 DENNIS