36 Tansley and Fritsch. 
Sonneratia it depends on external periodic mechanical factors. 
Westermaier points out that in Schimper’s description of the 
anatomy of the stilt-roots of Rhizophora there are indications that a 
similar respiratory mechanism occurs—a point which is well worth 
investigation, since it would be a very striking case of convergent 
development in two plants of different orders. 
Avicennia officinalis (Verbenaceae) is another characteristic 
mangrove, forming a bush or small tree with oval pointed leaves 
covered underneath with a white down which gives the foliage a 
greyish tint. It has aerial roots or pneumatophores of exactly the 
same type as Sonneratia, though it belongs to a totally different 
family, but their cortex communicates with the air by means of 
lenticels. This plant is also viviparous, but the development of the 
embryo does not proceed nearly so far as in the Rhizophoracea?. 
The whole fruit falls off the tree, the hypocotyl elongates a little, 
and lateral roots sprout from its swollen lower end, very quickly 
fixing the seedling in the mud. The cotyledons are fleshy and 
folded, and lie on the mud or are raised a little above it. It is a 
very widely distributed and common species in the East, and 
in protected places often forms pure formations on the water’s 
edge. It is common in Ceylon, according to Trimen, but we saw 
very little of it (except at Negombo, where it forms a marked 
feature), though a stunted bush of this species was the only 
individual mangrove that we met with on the actual sea-shore. 
This was at Tangalla, where the bush in question was growing quite 
isolated, and certainly far from the nearest individuals of the 
species. It was evidently having a hard time, in a little shallow 
mud and sand w r hich had collected in the hollow of a rocky 
promontory. The little patch of mud was nevertheless full of the 
characteristic upright roots. 
Acanthus ilicifolius is the only member of the true mangrove- 
flora which is herbaceous. It forms low bushes only two or 
three feet high, with leaves exactly like those of the holly, and beauti¬ 
ful lilac-purple flowers of the ordinary Acanthus-type. This plant 
never grows where it is exposed to much tidal action, but is 
extremely common lining the front of the mangrove formation on 
the edges of the estuaries and lagoons. It is in fact the most 
characteristic waterside mangrove after Rhizophora itself, forming 
a dense belt round many of the little islands in the estuaries, with 
Sonneratia, Druguiera, etc., behind (Fig. 12). Where the mangrove 
formation is well developed, Acanthus generally forms an abundant 
