506 JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Hence we recognize the difference between the timber of oaks and 

 pines (xerophytes), of elm (a mesophyte), and of willows and poplars 

 (hydrophytes). 



Stems in dry countries are often used as means of storage of water, 

 as in the Baobabs and "bottle-trees," &c. The pseudo-bulbs of 

 epiphytal orchids growing on lofty trees are means of storing water, 

 while the fleshy- stemmed Oactaceae of Mexico resemble the Euphorbias 

 and Stapelias, &c., of Africa in having the same function. The 

 swollen internoides of Poa hulhosa are formed only in dry places. 



The wood, as stated, increases in hardness so that what would be 

 a soft biennial herb in England becomes a stunted, gnarled stem of a 

 perennial, as in non-climbing species of Convolvulus, Heliotrope, &c. 



Leaves.' — The most characteristic xerophytic features are to be 

 seen in leaves. The parts of plants can only grow to a full size 

 when well supplied with water, as might be expected, and the leaves 

 are relatively reduced in size or may be arrested altogether, as in 

 Broom. A complete series can be found between the large-leaved 

 shrubby Veronicas of New Zealand and the Box-leaf-like form of 

 Veronica buxifolia to others resembling the Cypress, V. cupres si folia, 

 and the fleshy V. salicornioides just like our Marsh Samphire. 



The general characters of many xerophytes are mimetic. Some are 

 called '' ericoidal" when heath-like, as in members of the Daphne 

 family, like Pimelia. A leathery or coriaceous type is very common, 

 and as the shoots are equally starved with the leaves, they often 

 terminate in a hard and sharp point, as in our rest-harrow and furze. 

 To prove that it is nothing but drought vv^hich is the cause of these 

 peculiar habits of growth, if these and other spiny plants are grown 

 with plenty of water, both leaves and spines grow normally as in 

 ordinary mesophytes. 



Fleshiness is another common character. The object is to store 

 water, and it is seen in both stems and leaves. The Crassulaceae are 

 characterized by this type of leaf, as in Sedums. 



Very hairy and woolly leaves are common, as the edelweiss of 

 Switzerland, woolly species of plantains and Convolvulus, and occur 

 in the desert near Cairo. The hairs on many leaves act as absorbents 

 of dew in the hot, rainless season, and though, like the epidermis, 

 they are often coated with wax, they have " gashes " at the base so 

 that superficial water can pass through below. In some cases the 

 hairs swell into globular bladders filled with water, as in the ice-plant; 

 but the water-storage cells are generally in the interior, as in Reau- 

 muria, allied to Tamarix. Both of these secrete salts, which imbibe 

 much dew, which is absorbed and stored up within. There is a grove 

 of tall tamarisks near the barracks at Cairo, which flourish well, 

 though they are not artificially watered as the Lebeks (Acacias) are in 

 the streets. 



In compensation for the reduction in size of the leaves the 

 " palisade " cells below the epidermis are increased in number from 

 one layer or two to four and even five. 



