118 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



magnesium.* These excessively hygrometric salts absorb dew, which is 

 transmitted to the plant, and thus enable it to retain its bright green 

 colour all through the hot season. 



I have already alluded to wax as being frequently secreted on the 

 cuticle and epidermal hairs. This is a strong preventive to the loss of 

 water, which escapes as vapour through the stomata. 



It is the presence of wax and the thick coating of hair which obscures 

 the green chlorophyll, to which the blue-grey colour so observable in many 

 South African plants, and still more so in North African deserts, is due. 



'Wax is particularly present in such South African genera as Rhus, 

 Cotyledon, Protea, Myrica. Dr. Marloth thus describes the stems of 

 Sarcocaulon Burmanni and S. Patersonii, which " will burn even when 

 green, because the outer bark consists of cork well saturated with wax. 

 On the older branches this layer is about one tenth of an inch thick ; but 

 if one extracts the wax by repeatedly boiling it in chloroform it swells, 

 becoming ten or fifteen times thicker, and showing ten to thirty annual 

 rings, each consisting of numerous layers of cork-cells. On the plant, 

 however, they are glued together by the wax, and form a solid mantle 

 entirely enveloping the stem." 



As a continuous coating of wax would nullify absorption the bases of 

 the hairs of some plants in the northern deserts at least, which are more 

 or less coated with it, are often " slashed," the wax being wanting over 

 parts of the lowermost cells, through which water can pass within. 



Conclusion. — The universal features observable in the peculiarities 

 of plants adapted to drought are that they not only occur alike in various 

 members of the vegetable world, in precisely the same manner — as the 

 Cactacece and Euphorbia cccz so well show — but that they are the actual 

 result of the influence of the external conditions of life upon the plants. 

 These respond to those conditions, and have thus acquired all the vegeta- 

 tive features so characteristic of the general facies of the so-called 

 " xerophilous " plants. I repeat that it is one of the many illustrations 

 of what Darwin called the "definite action of the environment" under 

 which " new sub- varieties arise without the aid of natural selection." t 



* Such was the result of analyses, by the late Dr. E. Sickenberger of Cairo, of the 

 Halts in Rikiiunuria. 



t An. and PL under Dom. ii. p. 271. 



