312 



PROTECTIVE. ARRANGEMENTS ON- THE EPIDERMIS. 



to the sun's heat. One day, after a warm dry east wind had swept for only a short 

 time over the foliage, it became quite brown, and in the evening all the leaves were 

 entirely dried up and dead. And yet leaf -segments of this palm appear to be firm, 

 leathery, and dry, and one would have supposed them to be particularly well pro- 

 tected against drying up. The section of part of a leaf which is represented in 

 fig. 75, however, corrects this impression. This shows that the epidermal cells are 

 certainly very compact, by which the firmness of the leaf is materially increased, 

 but that their walls are not thickened, being only like those of a delicate fern in 

 this respect. Under these small thin- walled epidermal cells lie large succulent cells 

 which form the so-called aqueous tissue, the structure of whose walls likewise 

 cannot limit evaporation; below these are the large succulent cells of the green 

 tissue. A glance at this leaf section will make it clear that this palm is well 



Tig. 75.— Vertical section of a portion of the leaf of Caryota propinqiw; x 



adapted to its warm damp habitat, where it is never exposed to a strong evapora- 

 tion, but not to the dry, even if warm, air of a Continental climate. 



To the wax-like excretions of the cell-wall which form a delicate bloom, easily 

 rubbed off, on both sides of the leaf, frequently colouring it pale blue, grey, or white 

 instead of dark green, it has already been stated that the role is assigned of protect- 

 ing the stomata from moisture. From what has been said, one would expect that 

 these waxy coverings, which are especially to be met with in the Cruciferse and 

 Rutacese of steppes, in many acacias and Myrtaceae of Australia, and in the pinks 

 and spurges of the Mediterranean flora, would also be able to limit transpiration in 

 the epidermis — that is, in the structures over which the bloom-like covering extends. 

 Experiments specially undertaken, have also shown that in the same space of time, 

 and under otherwise similar conditions, leaves whose bloom had been carefully 

 rubbed off lost almost a third more water than others whose waxy covering had 

 been left intact. 



That the varnish-like covering of the epidermis, composed of a mixture of 

 mucilage and resin ("balsam"), which is excreted from capitate hairs and other 

 glandular structures, is able to restrict transpiration has also been pointed out. 

 These coverings are especially developed in many plants of the Mediterranean flora, 

 particularly in a whole group of Cistus (G. laurifoKus, populifolius, Glusii, ladani- 

 ferus, monspeHensis, &c); further in shrubby plants which develop late, in the 

 height of summer — as, for example, in Inula viscosa, which is so abundant on the 



