288 THE FLORA OF BRITISH EAST AFRICA part hi 



periods of drought. It is modified to enable it to survive the 

 hot dry season in four different ways, each based on the principle 

 of reducing the loss of moisture in " transpiration." The 

 simplest contrivance is that adopted by the h-A.oh-dhi^Adansonid)^ 

 the leaves of which remain on the tree only during the rainy 

 season. A second plan is used by many of the acacias, in 

 which the leaves are reduced to mere spines or needles, the 

 fleshy tissue being lost, and only the veins or "vascular bundles" 

 left. A third arrangement for the same end is the reduction 

 of the surface of the leaf in proportion to its mass ; thus the 

 leaf becomes thick and succulent, and the number of " stomata" 

 (or pores through which moisture can escape) lessened, as in the 

 fibre-yielding plants, such as the aloe and Sanseviera. The 

 last and extreme method is the entire disappearance of the 

 leaves, which are represented only by thorns and spines, while 

 respiration is effected by the green succulent stem. The loss 

 of moisture from the plant is therefore greatly reduced, for the 

 surface on which it can take place is small in comparison with 

 that exposed on a leaf-bearing tree. The plant secures, in fact, 

 a minimum of surface with a maximum of mass. The thick 

 succulent stems, moreover, contain special stores of moisture, 

 and reservoirs of milky juice or " latex," which, being confined 

 in special vessels or elongated cells (tracheides), can only escape 

 by the slow process of exosmosis. The plants in which this 

 adaptation has been developed are the most remarkable- 

 looking in East Africa. Such are the species oi Aristolochia, a 

 genus which has one representative — the birthwort {A. cleniatitis, 

 Linn.) — established in England ; the forms in the Nyika are 

 huge spherical bulbs, sometimes 3 feet in diameter, from 

 which long trailing branches, armed with thick spines, spread 

 over the ground. The bulb is full of juice ; this is reported 

 to be very poisonous, and the only animals that attack the plant 

 are the ants. The Spurges {EupJiorbiacecB) offer a still better 

 illustration, for they occur in two very different types. In the 

 grass plains, as on Laikipia, this family is represented by small 

 herbs, with leaves and structure like the common spurges of 

 our woods and fields ; but in the Nyika, the species (such as 

 Ejiphorbia NyikcB, Pax.) are lofty candelabra-shaped trees, from 

 30 to 60 feet in height, with thick succulent stems like 

 the cactus. In other places, as in the Baringo basin, a closely 



