290 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



of the uiiilulating margin. The suu's rays are refractuil as through a lens by this 

 raised water, and so a light stripe corresponding to each convex division of the 

 curved margin is formed on tlie bed of the lake, and a dark stripe corresponding to 

 each concave part. These are arranged in a radiating manner round the dark 

 central portion of the shadow. 



MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



Special arrangements are met with in all plants which possess stomata, in order 

 that the giving off of aqueous vapour may continue without hindrance. Water 

 falling on the upper side of the leaf, in the form of rain and dew, threatens to 

 cause the greatest olistacle to tliis free passage should it be able to collect directly 

 in the stomata. The width of an open stomate does not render the entrance of 

 water by capillarity impossible. As long as light and warmth exercise their power, 

 as long as the temperature in the neighbourhood of the spongy parenchyma is 

 higher than that of the surrounding air, and water-vapour in conse(iuence is pro- 

 duced in the s^^ongy tissue and driven out with force from the stomata, such an 

 entrance is indeed inconceivable. It is impossible for aqueous vapour to pass out 

 and at the same time for fluid water to enter by the same passage and through the 

 .same gate. But should the leaf become cooled by radiation after sunset, and dew 

 be dejDosited upon it, or should a cold rain trickle down over the leaves, and the 

 stomata have been unable to close (juickly enough, it is quite possible that water 

 might enter, just as it enters a retort (whose narrow mouth dips into water, and 

 whose contents have been vaporized by placing a lamp under them), when the lamp 

 is removed, and the bulb of the retort with its contents becomes cooled. But putting 

 aside the possibility of water thus pressing its way in, this much is certain, that 

 the formation of a layer of water over the cells in the immediate neighbourhood 

 of the stomata would cause great injury to the plants; and this, not only as affecting 

 transpiration, but also the free entrance and exit of gases. Therefore, the im- 

 mediate surroundings of the stomata must be kept open as a path for aqueous 

 vapour, and no water must be allowed to collect and take up a position there. 



Stomata are much too small to be seen with the naked eye. However, it can be 

 ascertained by a very simple experiment whereabouts, on a leaf or green branch, 

 stomata are to be found. A twig or a leaf is dipped in water, and then withdi'a\vn 

 after a short time and lightly shaken; some spots will be found wet, while otiier 

 places remain dry. Where water remains and spreads out to form an adhering 

 film, no stomata will be found in the epidermis ; but where the twig or leaf is dry, 

 one can be sure of finding them. In 80 per cent of cases experimented upon in 

 this way, only the upper leaf-surface became wetted, while the under side kept 

 dry; in 10 per cent both sides remained dry; and in the other 10 per cent the 

 upper side kept dry, while it was the under side which was wetted. With this 

 corresponds the actual fact that in far the greater number of instances the under 

 side possesses most stomata, while the upper side is free from them. It seems as if 



