35 
Flora of the Ceylon Littoral. 
shrub a few feet high to a tree which may attain as much as 50 
feet. Its leaves are obovate, of a light, rather dull green, forming a 
striking contrast to the dark glossy green of the Rhizophora- 
Bruguiera vegetation. The foliage is rather loose, and is disposed 
about the trunk and branches with something of the elegance, 
though not all the lightness, of the silver birch. 
Like all the true mangroves, Sonneratia has aerial roots for 
the purpose of allowing air access to the tissues of the part of the 
root-system which is buried in mud. Long horizontal roots run 
close below the surface of the ground, sending down much-branched 
rootlets deep into the mud, and sending upwards straight un¬ 
branched roots which rise a.foot or more above the surface. These 
upright aerial pneumatophores often form a perfect forest where the 
horizontal underground roots are numerous. Like the aerial parts 
of the stilt-roots of Rhizophora , and the knee-roots of Bruguieria, 
these aerial roots of Sonneratia possess a thick and richly lacunar 
cortex, the intercellular spaces of which are open to the air 
through the peeling of thin investments of cork (aerenchym). 
Trimen mentions that the aerial roots of Sonneratia are cut into 
slices and used by entomologists in their collecting boxes as a 
substitute for cork, owing to their spongy but firm and even texture. 
S. alba and 5. apetala, which have similar pneumatophores, are 
also found in Ceylon, but they are very rare, and we have not met 
with them. 
Westermaier has recently investigated the respiratory 
mechanism of these roots of Sonneratia, with interesting results. 
The cortex of the submerged and subterranean portions of the 
pneumatophores, as well as of the horizontal roots from which they 
arise, is highly lacunar; it lacks the characteristic supporting 
elements (trichoblasts) which occur in the rather less strongly 
developed cortex of the aerial part of the pneumatophore. In their 
place, however, it contains peculiar arc- or S-shaped cells 
(“ Federzellen ”) which are capable of compression and expansion 
like springs, and lead to alternate collapse and distension of the 
cortex in this region, as the water rises and falls with the change of 
tides, or when the water is disturbed. Each compression of the 
cortex of these submerged parts forces a certain amount of the 
contained air out through the aerenchym of the aerial part of the 
pneumatophore, whilst in the subsequent expansion a supply of 
fresh air is sucked in from outside. We thus have a respiratory 
process, which is quite analogous to our own, although in 
