DEPOSITS OF WAX. 



117 



Stance of the outer wall of the epidermis and the cuticle itself, partly on the 

 outside of the latter. These waxy deposits, first exactly investigated by De Bary, 

 appear in the form of exceedingly small granules on the surface of the cuticle, 

 or as fine rodlets, or lastly as continuous, occasionally very thick, crusts of wax, 

 which, in virtue of their fatty nature, do not allow water to adhere to the 

 epidermis; so that rain and dew hanging on the shoots usually form round 

 drops. If such leaves clothed with wax are dipped in water, they exhibit in it 

 a silvery-glancing layer of air, and after withdrawal, it is found that they have 

 not been wetted by the water, but are dry. 



By means of this arrangement, the internal tissue is protected against the 

 loss of those substances which would 

 diffuse out through surfaces coming di- 

 rectly in contact with water — a point 

 to which I shall return at another 

 opportunity. It is obvious, by the way, 

 that the arrangements just described are 

 wanting in subterranean and submerged 

 shoots which absorb water and nutritive 

 materials from without. 



Although the leaf-surfaces are pro- 

 tected in the manner stated against the 

 entrance of water from without, as well 

 as against the unlimited evaporation 

 of the water of the cell-sap; there yet 

 remains on the other hand the necessity 

 of letting aqueous vapour escape according 

 to circumstances from the assimilatory 

 siufaces into the atmosphere, in order that 

 fresh water containing nutritive materials 

 absorbed by the roots may follow on into 

 the organs of assimilation; and it is like- 



. J tr cane). ^ internoae 



Wise necessary to provide passages tor surface. (Desary.) 

 the entrance of carbon dioxide as well as 



the exit of the oxygen gas produced by its decomposition. We find these in the 

 so-called . stomata of the epidermis. The more vigorous the transpiration, and, 

 consequently,, the more copious the absorption and decomposition of carbon 

 dioxide by means of the chlorophyll-corpuscles, the more numerous in general 

 are the stomata. Hence it is pre-eminendy on the surfaces of the green organs of 

 assimilation, the foliage-leaves, that they occur in enormous numbers; while they are 

 always wanting at the roots, and are only found on subterranean and suWerged 

 shoots incidentally and in small numbers. On the sub-aerial shoot-axes and other 

 organs they are more or less numerous according to circumstances. The extra- 

 ordinary minuteness of the stomata, in connection with the properties of the cuticle 

 which clothes them, prevents water under ordinary circumstances from entering 

 by capillarity, and likewise prevents dust, fungus-spores, &c., from penetrating 

 their openings; while an adequate exchange of gases and discharge of aqueous 



FIG. 116.— Epidermis and layers of tissue immediately 

 subjacent, of the haulm of Sacckarum officinarum. {Sugar- 

 cane). A intemode ; B node — showing wax formations on the 



