MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



295 



busoides, which so much resemble the bamboos, besides some sedges (e.g. Carex 

 pendula), exhibit on the other hand the above-named peg-like pi-ojeetions of the 

 cuticle; these are shown in the section of a bamboo leaf in fig. 66 2 ). On plunging 

 such a bamboo leaf in water, a surprising sight presents itself. The upper side, 

 covered by a dark green, smooth, flat epidermis, with no stomata, becomes wet all 

 over, and retains its dark colour and dull appearance; but the under surface, blue- 

 green in colour, and beset with stomata and thousands of cuticular pegs, does not 

 allow the air to be displaced; and this layer of air, spread thin over the surface, 

 glistens under water like polished silver! The leaf may be shaken under water 

 to any extent, and may even be left submerged for a week, but the silveiy glisten- 

 ing air-stratum is not dislodged. If such a leaf is now taken out of the water, the 

 upper surface is quite wet, but the under surface is dry, like a hand which has 

 been dipped in mercury and then withdrawn, and not the smallest drop of water 



Wo dSpj 



y Papilla-like outgrowths of the Surface. 





Fig. 65. — Protection of Stomata from Moisture by Papilla-like outgrowths of the Surface 



1 Vertical section through a portion of the leaf of Glyceria spectabilis. a Vertical section through a portion of the leaf of 



Carex paludosa ; x200. 



adheres to it. On placing a vessel of water, in which some bamboo leaves are 

 half immersed, under the receiver of an air-pump, and then pumping out the air, 

 numerous small air-bubbles are at once given off from the submerged portions 

 of the leaves. At length the silvery lustre disappears, and the air between the 

 cuticular pegs is replaced by water. If now the leaves be completely submerged, 

 the silver lustre is only shown on those parts which were not previously immersed, 

 and where water could not replace the exhausted air; — the spaces round the pegs 

 in this region having been again supplied with air on the opening of the stop-cock 

 of the pump in order to submerge the leaves. It may be imagined from this 

 experiment how much the stomata would be damaged by water if the plants 

 mentioned were not protected from moisture by the pegs to which the air adheres 

 so strongly. 



In many plants which grow in the sunshine, and particularly in those whose 

 foliage is evergreen and only exposed to moisture at the time of the greatest 

 activity of the sap (while later it is exposed for months to dry air), the stomata are 

 to be found surrounded by an embankment, or sunk in special pits and furrows. 

 Even in the leaves of many indigenous plants, which are green in the summer, 

 e.g. those of the Carrot (Dmicus Caroia), the guard-cells of the stomata are so 



