Deserts 
socontrived as to be thoroughly efficient. In the Egyptian 
desert, near the Pyramids and about the level of the 
highest inundation of the Nile, a small thistle is fairly 
common. It(Carduus acaulis) consists of a small rosette 
of leaves which lies flat on the ground, and a head of 
flowers with no stalk but seated in the centre. Each bract 
of its involucre is continued into a long yellow very sharp 
spine. They are so arranged that the sensitive nostrils 
or tongue of a browsing animal must somehow dispose 
of them before the green leaves can be touched. 
The wait-a-bit thorn (Acacia detinens) has also a very 
ingenious yet simple arrangement in its stipule thorns, 
They are in pairs ; one is straight and pierces the nose 
of an animal, and the other is crooked and would catch 
in its tongue. 
This genus, Acacia, has a very interesting part to play 
in the economy of most dry countries. It is the rule to 
find all deserts surrounded by a broad or narrow belt of 
thorny thickets or open thorn-woods in which Acacias 
or allied forms are particularly common,® This fringe 
of Acacias seems to be present along the whole southern 
edge of the Sahara, from the Atlantic Coast to Egypt, and 
forms a belt 375 miles wide between Tuat and Gao on 
the Niger.° It is found in Northern Africa wherever 
the incessant demands for firewood have allowed it to 
persist. In South Africa, in Chile, and in Australia a 
similar belt seems to be present. For such a position 
it has many advantages. The long roots easily penetrate 
to depths of 20 or possibly 30 feet in search of water. 
Some Australian species have vertical flattened leaf- 
stalks which are placed edgewise to the sunlight, and in 
consequence are not likely to be injured by excessive 
heat, 
Many of them on_ being wounded produce gums such 
as gum-arabic and senegal, which, flowing out profusely 
177 M 
