124 ACACIAS IN VARIOUS PLACES. [ Sess, Lxx. 
420 species. Why it should appear so regularly near but not 
quite over true deserts seems to depend on the following 
adaptations :— 
1. The roots are usually very long, twenty to thirty feet in 
some cases, So as to reach deep-seated water. 2, The leaves are 
generally protected against strong sunlight by special devices, 
phyllodes, special powers of movement, etc. 3. Grazing 
animals are kept off usually by stipule spines, or, especially 
in the Australian species, by the development of tannin in 
the bark. Some species are extremely hardy: A. Greqgii 
(the one used for the lac-insect) is able to do with only three 
inches of rainfall in the year; A. longifolia is a sand-dune 
plant. 
Now in a general way, when one passes from an ordinary 
tropical “monsoon” wood into a desert, the vegetation 
changes as follows :— 
First, the ordinary, close-ranked array of the tropical wood 
becomes altered into a light, more open wood. Second, the 
trees separate, forming clumps or patches, as one finds in an 
English park. Third, the trees become thorn-trees. Fourth, 
the thorn-clumps scatter and become scrub or thickets of 
thorny bushes. Finally, the thickets open out into isolated 
pioneer thorn-shrubs or small trees dotted over the ground 
like the plants in an orchard. 
These isolated pioneers or scouts are almost invariably 
acacias, whilst the proportion of acacias and certain other 
Leguminose diminishes gradually in the vanguard of serub 
and advanced guard of thorn woods until in the true “mon- 
soon” wood there are exceedingly few or none at all. 
This sort of succession can be seen in a great many places. 
Even in the Mediterranean region A. Farnesiana thrives and 
is even cultivated for its flowers (100,000 lbs. of essence has 
been made at Grasse). 
In Egypt their importance is at once manifest. As one 
slowly steams up the Nile between sloping mudbanks a few 
feet high and covered with lupines and Lubia beans, the 
oniy vegetation above the bank consists of acacias or «n 
occasional line of tall, graceful date-palms. 
On landing, one finds a perfectly distinct line which shows 
the limit of the last inundation of the Nile. Beyond this 
line, acacias are almost the only shrubs or trees. They also 
