FOLIAGE AND SCALE LEAVES. 6g 



spring water. If now the apparatus is placed in bright sunlight, 

 bubbles of gas will escape from the cut end. These are easily 

 collected in a test-tube, and can then be proved to consist of 

 oxygen by the usual methods, e.g., re-ignition of a glowing match- 

 end plunged into the tube. The leaves (and other parts to a less 

 extent) of a land plant are constantly giving off a large amount 

 of aqueous vapour into the air ; in other words, they transpire. 

 This transpiration is made up for by the active powers of absorp- 

 tion that the root possesses. Numerous familiar facts receive 

 their explanation in this, such as the fading of cut leaves and 

 flowers (p. 9). The loss of water experienced by these makes 

 their cells lose turgidity, and this causes limpness. That water 

 is actually given off on the one hand and taken up on the other 

 is easily demonstrated. A plant growing in a food solution, the 

 surface of which is guarded from evaporation, is covered with a 

 bell-jar, and placed under ordinary conditions of light and heat. 

 Two things soon become evident — aqueous vapour is given off 

 from the plant and condenses on the inner side of the glass, and 

 the food solution gradually diminishes in quantity. The amount 

 of transpiration depends upon a number of conditions. It is 

 naturally greater in dry than in damp air, and in the sun than 

 in the shade. The latter point is shown by a simple experiment. 

 A number of vigorous stalked leaves are collected. Half of these 

 are placed with their stalks in one glass of water and the other 

 half similarly disposed in another glass. It is convenient to 

 employ perforated cards, through the holes in which the stalks 

 are passed. If dry glasses are now inverted over the two lots of 

 leaves, and these are placed in the sun and shade respectively, 

 transpiration will proceed vigorously in the former case, but 

 much less so in the latter. ■ This is shown by the fact that, after 

 the lapse of some ten minutes, abundance of moisture will have 

 condensed on the inner side of the glass which covers the sunned 

 leaves, while that covering the shaded ones remains almost or 

 quite clear. The amount of transpired water may be very con- 

 siderable. It has been shown that a sunflower plant presenting 

 5616 square inches of leaf-surface loses in this way, on an average, 

 twenty ounces by weight of water during a day of twelve hours. 

 The corresponding loss at night is only about three ounces. 

 Transpiration in the mature leaf takes place partly from the 

 general surface and partly by means of the stomata. The former 

 (" cutictdar") transpiration is greatest in herbaceous leaves with 

 a thin cuticle, and least in leathery leaves where the outer cell- 

 walls of the epidermis are thickened and cuticularized and covered 

 by a well-developed cuticle. Stomatal transpiration is much more 

 important, and, in fact, the chief use of stomata is to effect this. 



