PECULIARITIES OF THE CAPE FLORA. 



117 



In some plants of the Composites the spinescence is confined to the 

 scales of the involucre, as in the Calthrop (Centaitrea Calcitrajja). 



In some it is the arrested branch which constitutes a spine, as in our 

 own Furze, but the leaves of this are also spinescent. A common tree in 

 South Africa, called the Kei Apple tree {Abelia), is often used for hedges 

 on account of its formidable sharp-pointed branches. 



The Epidermis. — As the chief object is to prevent the loss of water by 

 transpiration, it is the surface of the leaf which acquires various peculi- 

 arities to secure this result ; consequently the epidermis acquires a thick 

 cuticle composed of suberine, i.e. of a corky nature. Wax is often secreted 

 on the surface as well as on hairs arising from the epidermis. Silica 

 may also be present ; while the stomata are variously protected, either 

 by being sunk in deep pits clothed with hair or with other obstructions, 

 as water-cells, on the surface. Moreover it has been found that they 

 close if the heat be very excessive, whereas in temperate climates they 

 open on a warm sunny day. Again, the surface may be covered with 

 silky hairs, as in the leaves of the ' Silver Tree,' or woolly, as on some 

 of the many ' Everlastings ' of the Cape. 



In some cases the hairs are "stellate" or consist of "needles" 

 flattened so as to make a felt-like surface, acting as a non-conductor. 



In some the epidermal cells act as water-storage, or the hairs 

 themselves may be globular and full of water held in reserve, as is easily 

 seen on the 'Ice-plant ' (Mesembryanthemum crystallinum). 



The hairy covering, while acting as a preventive against loss of 

 water, is also an absorbing instrument during the hot season, when there 

 are excessively heavy dews at night. 



That plants can absorb water as rain or dew by their foliage, whether 

 hairy or not, has been satisfactorily proved by Boussingault and others, 

 including myself. The following are a few examples out of many in a 

 series of experiments with dew. 



If a leaf is quite turgid it is not likely to absorb any appreciable 

 quantity, so I took some score of detached leaves of different English 

 plants and laid them in the sun for three hours on a September afternoon 

 until they were all more or less flaccid. I then weighed them, placing 

 them on a close-cut lawn after sundown. They were left cut all night 

 to a heavy dew until just before sunrise. I then carefully removed all 

 superficial moisture. 



Besides their being obviously and completely "freshened up," the 

 gains per cent, in weight was most pronounced.* 



Hairy leaves have an advantage over smooth ones, as the hairs can 

 not only absorb better than a thin cuticle, but retain the drops of water 

 for a much longer time than a smooth surface alone can remain wet. 



Another means of absorbing dew, according to Dr. Volkens and 

 Dr. E. Sickenberger, occurs in certain desert plants, such as Tamarix (and 

 its herbaceous ally Beawmiria in North Africa), Frankenia, of which 

 South Africa has three species, and Stat ice. They secrete, by the 

 evaporation of water at the surface, chlorides of sodium, calcium, and 



* See " On the Absorption of Rain and Dew by the Green Parts of Plants," Journ. 

 Linn. Soc, Bot. xvii. 1878, p. '6V6. 



