142
WILLIAM MACNAE
pressure of 23-25 atm (this is normal for sea water). On the other hand,
Avicennia marina varied between 35 and 46 atm. Sen Gupta (1938)
found similar values in the Ganges delta (at the mouth of the Haringhanga river) listing Aegiceras corniculatum with 38.2 atm, R. mucronata
32.1 atm, Avicennia marina 25.9 atm, Ceriops decandra 26.7 atm,
Lunznitzera racenzosa 41.9 atm, and Xylocarpus granatum with 39.9 atm
against a water analysis of 9.84 atm. Whether the osmotic pressure
shows variation with varying salinity of the soil does not seem to have
been investigated.
According t o Cooper and Pasha (1935) the suction pressure or
hydrostatic pressure follows the values of the osmotic pressure. They
record suction pressures 60-82 atm in the stem of Xonneratia apetala at
Bombay against osmotic pressures of 60-85 atm with a soil water
osmotic pressure of around 25 atm.
The discrepancy in measurements of osmotic pressures seems to
point to a need for further investigations.
Salt excretion occurs in species of Avicennia, in Aegiceras corniculatun?, and in Acanthus ilicifolius through epidermal glands (Fig. 40) of
very similar form in the three genera. Those of Aegiceras were first
described by Areschoug (1902a,b) and by Schmidt (1904a). Mullan
(1931a) gave descriptions of all of them and pointed out that these
glands occur on both leaf surfaces and on the petioles as well. He also
reported that they are more abundant on the leaves of trees and bushes
growing in hypersaline areas and absent from plants of Acanthus
ilicifolius growing in fresh water. Ruhland (1915) studied them in
Aegialitis unnulata, and Walter and Steiner (1936) those of Avicennia
marina.
Scholander et al. (1962) have given some attention to the salt
balance in mangrove trees. They have shown that the xylem sap of
species bearing salt glands (Aegialitis annulata, Aegiceras corniculatum
and Avicennia marina) had a concentration of 0.2-0.5% sodium
chloride. The xylem sap of Rhizophora mucronata which does not have
salt glands contained one-tenth of that amount and Hibiscus tiliaceus
less than one-hundredth. They give no comparable figure for Sonneratia,
the wood of which is said to corrode iron nails as a result of its salt
content.
FIG. 40. Diagrams to illustrate the leaf anatomy of Sonneratia (A, B), Rhizophora
(C, D) and Awicennia (E). I n the older leaves of Sonneratia and Rhizophora (B and
D) the water storage tissue is much increased in extent by increase in size of cell.
(Partly after Walter and Steiner, 1936.) Diagrams to show salt-excreting cells of
Aconthus (F), Aegiceras (G) and Avicennia (H). There is a close resemblance of the
form of the glands in these genera. (Partly after Mullan, 1931a.)
WILLIAM MACNAE
pressure of 23-25 atm (this is normal for sea water). On the other hand,
Avicennia marina varied between 35 and 46 atm. Sen Gupta (1938)
found similar values in the Ganges delta (at the mouth of the Haringhanga river) listing Aegiceras corniculatum with 38.2 atm, R. mucronata
32.1 atm, Avicennia marina 25.9 atm, Ceriops decandra 26.7 atm,
Lunznitzera racenzosa 41.9 atm, and Xylocarpus granatum with 39.9 atm
against a water analysis of 9.84 atm. Whether the osmotic pressure
shows variation with varying salinity of the soil does not seem to have
been investigated.
According t o Cooper and Pasha (1935) the suction pressure or
hydrostatic pressure follows the values of the osmotic pressure. They
record suction pressures 60-82 atm in the stem of Xonneratia apetala at
Bombay against osmotic pressures of 60-85 atm with a soil water
osmotic pressure of around 25 atm.
The discrepancy in measurements of osmotic pressures seems to
point to a need for further investigations.
Salt excretion occurs in species of Avicennia, in Aegiceras corniculatun?, and in Acanthus ilicifolius through epidermal glands (Fig. 40) of
very similar form in the three genera. Those of Aegiceras were first
described by Areschoug (1902a,b) and by Schmidt (1904a). Mullan
(1931a) gave descriptions of all of them and pointed out that these
glands occur on both leaf surfaces and on the petioles as well. He also
reported that they are more abundant on the leaves of trees and bushes
growing in hypersaline areas and absent from plants of Acanthus
ilicifolius growing in fresh water. Ruhland (1915) studied them in
Aegialitis unnulata, and Walter and Steiner (1936) those of Avicennia
marina.
Scholander et al. (1962) have given some attention to the salt
balance in mangrove trees. They have shown that the xylem sap of
species bearing salt glands (Aegialitis annulata, Aegiceras corniculatum
and Avicennia marina) had a concentration of 0.2-0.5% sodium
chloride. The xylem sap of Rhizophora mucronata which does not have
salt glands contained one-tenth of that amount and Hibiscus tiliaceus
less than one-hundredth. They give no comparable figure for Sonneratia,
the wood of which is said to corrode iron nails as a result of its salt
content.
FIG. 40. Diagrams to illustrate the leaf anatomy of Sonneratia (A, B), Rhizophora
(C, D) and Awicennia (E). I n the older leaves of Sonneratia and Rhizophora (B and
D) the water storage tissue is much increased in extent by increase in size of cell.
(Partly after Walter and Steiner, 1936.) Diagrams to show salt-excreting cells of
Aconthus (F), Aegiceras (G) and Avicennia (H). There is a close resemblance of the
form of the glands in these genera. (Partly after Mullan, 1931a.)