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few stomata, as we see in the olive tree and many acacias, 

 transpiration is small in quantity, and such plants are most 

 frequent in arid regions. A dense woolly covering of hairs 

 also resists drought, the fine cellular hairs absorbing dew, 

 and in dry weather forming a flat coat which resists tran- 

 spiration. The corky thickening which all epidermal cells 

 undergo seems a wise provision to prevent exhalation, which 

 is thus carried on solely by the stomata. 



Besides water, the leaves of plants give off gases, and thus 

 are most important organs, as it is on this wise provision of 

 nature that we and all the animal creation depend for the 

 purity of the air we breathe. Priestley first discovered that 

 plants enclosed in air vitiated by breathing restored it to its 

 original purity; then Ingenhousz proved that plants give 

 out oxygen, but only during daylight, that it is greatest in 

 sunshine, and ceases at night, that poisonous plants behave 

 in this respect like harmless, that it is from old leaves and 

 the under surface that the largest amount is eliminated, and 

 that aquatic plants give off the most. 



De Saussure determined that although carbonic acid was 

 consumed during the day, it was given off at night ; and that 

 Fungi, and blanched parts of plants, as well as leafless plants, 

 exhaled carbonic acid. The carbonic acid appears to be 

 manufactured by vital processes going on within the plant, 

 and not by direct combination of the carbon of the plant 

 with the oxygen of the air. It is believed that some nitrogen 

 is also given off by plants, no doubt resulting from the de- 

 composition of absorbed ammonia. The sap absorbed by the 

 roots, containing inorganic matters in solution, reaches the 

 leaves and is exposed to air and light, water is transpired 

 extensively, and the inorganic matter not required for the 

 growth of the plant and its secretions is stored up in the 

 leaves, increasing in quantity with their age, as we find by 

 the weight of ash left after burning them ; this consists of 



