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cording to the special organization of the plant and of its 

 separate parts. First : TextiLre of the plant. The stems of 

 most woody plants and trees are, as we have seen, almost en- 

 tirely prevented from transpiring by thick layers of cork ; and 

 transpiration is small in such stems and leaves as are covered 

 with a thick cuticle, such as the stems of Cacti, the leaves of 

 Agave and of Begonia, and hard evergreen leaves. The 

 leaves of Agave and Begonia, when cut off from the parent 

 plant, wither slowly, and can live for a long time without any 

 water, since evaporation is very slow, as the texture of the leaf 

 almost eliminates transpiration. These plants can also thrive 

 in a very dry soil. Tender leaves, on the contrary, in which the 

 cuticle is but slightly developed, as e. g. those of the Tobacco 

 (Nicotiana tabacum) and the Pumpkin, wither as soon as they 

 are removed from the plant, or if the soil becomes too dry. 

 This condition is also seen in many aquatic plants, such 

 as the Potamogetons, where the texture of the leaves is 

 very flimsy and weak, and as a consequence evaporation on 

 exposure to the air is very rapid indeed. Secondly, the age of 

 the leaf apart from its texture, exercises a very important in- 

 fluence on the activity and amount of transpiration in a given 

 time. This has formed the subject of recent investigations by 

 an Austrian observer, Von Hohnel. From these researches it 

 appears that we can represent the amount of transpiration at 

 the various periods in the life of a leaf by means of a curve. At 

 the time of its first appearance, the amount of transpiration at- 

 tains an expression very considerably greater than we find to be 

 the case at any subsequent period in the history of the leaf. 

 From this point the curve descends, indicating a diminution in 

 the amount of transpiration; after a time, however, the curve 

 begins to rise again, indicating an increase in the amount, 

 though it never attains its initial height, and then at length it 

 finally sinks gradually, indicating a marked decrease. The 

 meaning of this curve is very interesting. In their initial 

 condition the leaves had no cuticle and no stomata : — conse- 

 quently, whatever transpiration took place did so through 

 the closed cell-walls, forming the external surface of the epi- 



