Agriculture Reference
In-Depth Information
Fig. 2.5
Vivipary in mangroves (propagules).
a
Rhizophora stylosa
,
b
R. mucronata
3. Normal germination
4. Vegetative propagation.
Among these four means of reproduction, vivipary is the most significant. Vivipary
(Macnae
1968
; Gill and Tomlinson
1969
) means continuous development of the
embryo after fertilization while attached to the parent without any intermediate rest-
ing or dormant period. Therefore, the term 'propagule' is used for these embryos
instead of the term 'seed' (Fig.
2.5
). The development of embryo continues during
the dispersal by water (Van der Pijl 1972). It is argued that vivipary represents a pre-
condition for increasing salt resistance in the seedling thereby facilitating survival,
when detached, in a substrate of high salinity (Dagar et al.
1991
). In some mangrove
species such as
Aegiceras, Avicennia, Nypa
and
Pellicera
, a more advance state
of vivipary known as 'cryptovivipary' is found where the embryo emerges from
the seed coat but not the fruit before it abscises (Carey
1934
). The advantages of
vivipary are obvious for such plants which grow on the fringes of sea. After falling
from the tree, the propagules float and remain viable for considerably long period
of time. Mangroves have little or no capacity of vegetative propagation but some
species such as
Avicennia
and
Excoecaria
have the capacity to coppice and could
persist in western India in spite of over-exploitation (Blasco
1977
).
3 Salt Stress Regulation Mechanisms
Presence of salt water is not a physical requirement of mangroves (Bowman
1917
;
Rosevear
1947
; Egler
1948
). Mangroves are facultative halophytes occurring in
tidal areas where fresh water plants, which are intolerant to salt, cannot live. Most
mangroves are capable of growing in fresh water (Teas
1979
) but mangrove eco-
systems are not found in strictly fresh water environment, probably due to the fact
that mangroves are not good competitors and the salinity is important in reducing
competition from fresh water and terrestrial vascular plants (Kuenzler
1974
). Man-
