288 MEANS OF ACCELERATING TRANSPIRATION. 



propped up against the houses in the streets of our towns, their tops would reach to 

 the second story, and it would be possible to climb up to the windows by them as 

 if by the rungs of a ladder. Many of these palm leaves if placed in an upright 

 position would be equal in height to our forest trees. In all these leaves the 

 epidermis is only slightly thickened, the spongy parenchyma is well developed, 

 stomata are present in large numbers, and the surfaces of the leaves are so directed 

 towards the incident sunbeams that they are abundantly illumined and warmed 

 throughout. The leaves become decidedly heated by the sun's rays, and thus, even 

 in the saturated air of the tropics, the necessary amount of transpiration becomes 

 possible. Arrangements similar to those of the palms may be observed in the 

 Aroids and Bananas. These also develop their most extended leaves in the 

 saturated or almost saturated atmosphere on the banks of still or flowing water, 

 and in the moist heavy air of tropical primeval forests. 



It is obvious that means of increasing transpiration are required in those water- 

 plants whose roots are in the wet mud at the bottom of lakes and ponds, whose 

 stems and leaf -stalks are directly surrounded by water, and whose leaf -blades float 

 on the surface of the water, as for example the water-lilies (Nymphcea, Victoria), 

 the Frogbit {Hydrocharis morsus-rance), and the Nymphsea-like Villarsia (V. 

 nymphoides). The blade of the leaf is disc-shaped in all these plants, and the discs 

 lie side by side flat on the surface of the water. Frequently large areas of lakes 

 and ponds are covered with the floating leaves of these plants. The whole of the 

 upper side of such a leaf can receive the rays of the sun, and the leaf is thus 

 warmed and illuminated throughout. The under side of the leaf is coloured violet 

 by a pigment called anthocyanin, which we will consider more in detail later, and 

 of which it need only be mentioned now that it changes light into heat, and thereby 

 materially helps to warm the leaves. 



The aqueous vapour which is in consequence developed cannot escape below 

 from the large air-spaces which permeate the leaf, because the under side, which 

 floats on and is wetted by the water, possesses no stomata. The upper side is so 

 richly furnished with stomata that on 1 sq. mm. 460 are to be seen, and on a 

 single water-lily leaf about 2| sq. dms. in area, about 11^ millions. This upper side 

 alone provides a means of exit, and it is therefore important that the passage 

 should not be obstructed at the time of transpiration. If the rain should fall unre- 

 strainedly on the upper side of the floating leaves, the collected rain-water might 

 remain there for a long time, even while the sunbeams breaking through the clouds 

 after the shower are warming the floating leaves and inciting them to transpire. 

 In order to avoid this an arrangement is made by which it is rendered an 

 impossibility to wet the upper side of the floating leaves. The falling rain is formed 

 into round drops on reaching them, and does not spread over the leaf -surface so as 

 to wet it. But in order that the drops should not remain long on the leaves in 

 many of these forms, such as in the widely distributed water-lily {Nymphcea alha),ihe 

 leaf, where it joins the stalk, is somewhat raised, and the edges are bent a little up 

 and down in an undulating manner. This gives rise to very shallow depressions 



