THE BOTANY OF THE APPLE TREE. 15 



there is, therefore, a constant tendency of the water to pass out into 

 the air, and this would take place with great rapidity if it were not for 

 the impenetrable skin which covers both surfaces. This skin keeps 

 in the moisture of the leaves just as the oiled paper, which florists use 

 so much, keeps in the moisture of the flowers. But, as stated above, 

 there are breathing pores (or mouths) in the skin of the under side of 

 the leaf, and when the plant opens these for taking in or giving off 

 gases, some moisture escapes. When we remember how many of these 

 openings there are in a leaf we can readily understand how easily the 

 moisture can evaporate when they are open. 



Now this loss of water must be made good or the tree will perish. 

 When a leaf loses water its cells take moisture from the twigs; these 

 in turn take from the branches, and these again from the trunk, and 

 so on to the youngest roots which get their supply of water from the 

 soil. There is thus a movement of the water toward the point where 

 the loss occurs, and this movement is more or less rapid as the loss by 

 the leaves is more or less. In damp weather there is little or no loss 

 of water from the leaves, and as a consequence there is little movement 

 of water in the tree. In the winter, also, there is little movement of 

 the water, since evaporation is reduced to little or nothing. 



From what has been said it may easily be made out that the tree 

 has more water in it in the winter, when it loses little, if any, by evap- 

 oration, than in the summer, when its supply of water is constantly be- 

 ing reduced by the loss from the leaves, and careful experiments show 

 that this is actually the case. There is more water in a given bulk of 

 the tree trunk, or in the branches and twigs, in January and February 

 than in June and July. 



A word should be said here about the word "sap" as applied to the 

 watery part of the apple tree. It is simply the water in the tree in 

 which various substances are dissolved. Sap is not a living fluid; it 

 is not a kind of vegetable blood. It is more like the watery mixture 

 to be found in the alimentary canal of animals than like their blood, 

 and yet this is not a good comparison to make, as it is likely to lead 

 to false conclusions. Moreover, the sap is not a circulating fluid. It 

 does not pass up and down in currents like the blood currents of ani- 

 mals, and last, but not least, it does not go down into the roots in the 

 fall and come up in the spring. 



The growth of the tree which is the most obvious to the horticult- 



