Deserts 



so contrived 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 fiat 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 



