144
WILLIAM MACNAE
The Auid excreted was collected from bubbles of fluid which came
t o lie above the salt glands. This fluid had been prevented from
evaporating by coating the leaf with a film of grease. The fluid excreted
was found to be some ten to twelve times as concentrated as the sap
and higher than sea water on many occasions. They also point out that
excretion proceeds even when the leaves are coated with salt: " It
would appear that the glandular cells are capable of full activity, even
though in direct contact with a saturated brine ".
These workers also studied the hydrostatic or suction pressure and
the transpiration rate. For transpiration, average day-time rates came
out as follows: Aegialitis 5 mg/dm2/min, Aegiceras 2 . 5 , and Avicennia
6.5. Comparable values for non-salt-secreting forms were Rhixophora
2.5, Sonneratia alba 1-5, Lurnnitxera littorea 6.5, Hibiscus 6.5, and
Eugenia suborbicularis 7.5. Measurements of the hydrostatic pressure
suggested that salt separation requires an energy source and active
transport, since the hydrostatic pressure is insufficient to allow separation by ultrafiltration.
C. Relationship between the mangrove root and shoot systems
Avicennia marina would appear to be not only the most geographically widespread mangrove species occurring in the Indo-west-Pacific
but also capable of growing at any level within the mangal, unless, of
course, it has been superseded by another species. To what is this
wide perspective in distribution due? After accompanying me through
various mangals in the vicinity of Lourenqo Marques, Dr. P. Myerscough of the Botany Department, Edinburgh University, made a
suggestion which may be worth following up. To quote from his letter:
Avicennia marina seems capable of growing under the least favourable
nutritional conditions, as well as under the most favourable-if not ousted
from them by other species. It seems to be the most plastic species in its
growth, apparently capable of producing a widely ranging root system under
poor condit,ions [Figs. 14 and 151 together with a relatively sparsely leaved
and slow growing and therefore undemanding shoot system ; undemanding
both on nutrients and water transpired. Bruguiera gymnorhim on the other
hand may be the least plastic species, it appears to have an intensely crowded
root system which is not wide ranging and a top growth which is leafy,
casting a deep shade, and also relatively rapidly growing and so high
demanding. In other words it has a growth pattern geared to make best use
of optimal nutritional conditions, shading out would-be competitors above
ground and crowding out their roots below. But these have poor survival
value in poor conditions.
Nothing of this has been tested experimentally but my familiarity
with the distribution of mangrove species in the mangals along the
WILLIAM MACNAE
The Auid excreted was collected from bubbles of fluid which came
t o lie above the salt glands. This fluid had been prevented from
evaporating by coating the leaf with a film of grease. The fluid excreted
was found to be some ten to twelve times as concentrated as the sap
and higher than sea water on many occasions. They also point out that
excretion proceeds even when the leaves are coated with salt: " It
would appear that the glandular cells are capable of full activity, even
though in direct contact with a saturated brine ".
These workers also studied the hydrostatic or suction pressure and
the transpiration rate. For transpiration, average day-time rates came
out as follows: Aegialitis 5 mg/dm2/min, Aegiceras 2 . 5 , and Avicennia
6.5. Comparable values for non-salt-secreting forms were Rhixophora
2.5, Sonneratia alba 1-5, Lurnnitxera littorea 6.5, Hibiscus 6.5, and
Eugenia suborbicularis 7.5. Measurements of the hydrostatic pressure
suggested that salt separation requires an energy source and active
transport, since the hydrostatic pressure is insufficient to allow separation by ultrafiltration.
C. Relationship between the mangrove root and shoot systems
Avicennia marina would appear to be not only the most geographically widespread mangrove species occurring in the Indo-west-Pacific
but also capable of growing at any level within the mangal, unless, of
course, it has been superseded by another species. To what is this
wide perspective in distribution due? After accompanying me through
various mangals in the vicinity of Lourenqo Marques, Dr. P. Myerscough of the Botany Department, Edinburgh University, made a
suggestion which may be worth following up. To quote from his letter:
Avicennia marina seems capable of growing under the least favourable
nutritional conditions, as well as under the most favourable-if not ousted
from them by other species. It seems to be the most plastic species in its
growth, apparently capable of producing a widely ranging root system under
poor condit,ions [Figs. 14 and 151 together with a relatively sparsely leaved
and slow growing and therefore undemanding shoot system ; undemanding
both on nutrients and water transpired. Bruguiera gymnorhim on the other
hand may be the least plastic species, it appears to have an intensely crowded
root system which is not wide ranging and a top growth which is leafy,
casting a deep shade, and also relatively rapidly growing and so high
demanding. In other words it has a growth pattern geared to make best use
of optimal nutritional conditions, shading out would-be competitors above
ground and crowding out their roots below. But these have poor survival
value in poor conditions.
Nothing of this has been tested experimentally but my familiarity
with the distribution of mangrove species in the mangals along the