THE ANGIOSPERMAE : LEAVES 1005 



The protective capacity of the cuticle is sometimes enhanced by a coating 

 of wax, which produces the greyish film called popularly " bloom " and 

 known botanically as a glaucous surface. This waxy covering is independent 

 of the cuticle and is apparently excreted through it from the living cells. It 

 commonly consists of very minute grains, loosely adherent to the cuticle, 

 but in some Grasses and related plants it takes the form of slender rods 

 attached to the surface by their ends. A third type of waxy covering, in a 

 continuous sheet, occurs especially in Palms. The Wax Palm, Ceroxylon 

 (Uidicohim, produces a layer nearly 5 mm. thick, which is scraped off and used 

 to make candles, a single tree frequently producing as much as 25 lb. of 

 somewhat resinous wax. These waxy coverings are of considerable value as 

 a protection against evaporation, not only as an addition to the cuticular 

 covering but also by building up protective ridges round the stomata. In 

 many cases the covering, if rubbed off, can be regenerated. 



The outer surfaces of epidermal cells are usually more or less convex, 

 and in many plants they are markedly domed or even conical. The latter 

 type of extension of the cell, when well developed, produces a velvety surface, 

 which is of common occurrence among tropical jungle plants. An ecological 

 advantage has been attributed to this type of surface, in that water falling on 

 the leaf tends to spread by capillarity and hence to dry more quickly, thus 

 offering less encouragement to the growth of epiphyllous Lichens and 

 Bryophyta. The idea, though often quoted as an example of adaptation, 

 rests only on supposition and should not be accepted uncritically. 



The epidermis of the leaf bears trichomes of the same types as those 

 borne by the epidermis of the stem, though often in greater abundance, 

 especially on the lower surface. When only a few scattered hairs are present 

 the leaf surface may be described as hispid (Fig. 992), whereas a closer 

 covering is spoken of as tomentose or lanate (Fig. 993). Such densely woolly 

 coats as are implied by these latter terms may be influential in reducing 

 evaporation from the stomatal openings and they are, in fact, commonest in 

 species which habitually grow in exposed places and dry soils, but it must 

 not be inferred that they are confined to such plants or that all plants with a 

 woolly covering stand in need of such additional protection. Only direct 

 experimental evidence can justify such a conclusion. What experience in 

 cultivation does bear out is that woolly plants are usually intolerant of pro- 

 longed damp, especially in winter. The thick hairy covering holds moisture 

 tenaciously and probably facilitates fungal and bacterial attack on the tissues. 



A number of plants, such as Platanus orientalis, Timilago farfara, and 

 Verbascmn olympiaim, have only temporary coverings of hairs on the young 

 leaves, the hairs later disarticulating at their bases, sometimes at a pomt 

 marked by a specially thin zone of the cell wall, and fall oft" or are rubbed 

 off, leaving the mature leaf surface bare. Some other types of hairs are shown 

 in Figs. 994, 995 and 996. 



The mechanical strength of the epidermis may be very serviceable in 

 preventing the tearing of the lamina, and this is generally due to the cuticle, 

 which not only forms a continuous sheet binding the individual cells together. 



