296 



MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



over-arched by the neighbouring epidermal cells that a sort of vestibule is formed 

 in front of the true pore. It can easily be imagined that drops of water which 

 come to such places are not able to press out the air from this vestibule, and there- 

 fore cannot penetrate to the guard-cells of the stomata. In Hakea jiorida and 

 Protea mellifera, two Australian shrubs (see fig. 67), similar arrangements are met 

 with, but here the stomata- are still more over-arched, so that they are only visible 

 to anyone looking at the surface of the leaf through small holes at the top of the 

 dome. The stomata on the green branches of various species of Ephedra are 

 surrounded by mound-like projections from the cuticle of neighbouring epidermal 

 cells, and are at the same time somewhat sunken, so that an urn-shaped space is 



Fig. 66. —Protection of Stomata from Moisture by Cuticular Pegs. 



' Vertical section of a Bamboo leaf ; xlSO. 2 Part of the lower portion of the section; X460. » Part of the upper 



portion of the section ; x 460. 



formed above each stoma, from which water cannot dislodge the air. On the 

 leaves of Dryandra floribunda, one of the Proteacese which grows in the thick 

 Australian bush, several stomata occur at the bottom of small pits on the 

 under side of the leaf, and from the side walls of the depression spring hair- 

 like structures which interlace and form a loose felt-work, easily penetrated 

 by gases but not by fluids (fig. 68). The stomata on the leaves of the Oleander 

 (Neriwm Oleander) are similarly situated. These also are at the bottom of deep 

 pits on the lower side of the leaf, and the entrance to them is beset with extremely 

 delicate hair-like structures (see fig. 73 3 ). The oleander fringes the banks of 

 streams in the sunny open country of Southern Europe and the East, and in its 

 natural position it is most exposed to wetting by rain, mist, and dew, just when 

 transpiration is an absolute necessity for it. But even when the leaves are covered 

 on both sides with a layer of moisture, none can force its way into the hair- 

 lined depressions which conceal the stomata, and consequently transpiration is not 

 hindered even in the wettest season of the year. 



