342 STATE POMOLOGICAL SOCIETY. 



upon tlie tree. Of this external cause we liave an illustration in the maple 

 Avheu it is tapped and the spile is driven in. The sap, in its endeavor to fill the 

 vacuum makes its escape, and will continue to do so until the supply is 

 exhausted or the temperature falls. 



So, when the weather becomes colder, the air and water contained in the 

 cells contract, and the flow of sap ceases, but when the sun again shines forth, 

 "with refulgent ray," warming and enlivening every living thing, the air and 

 water are again expanded, and rushing out of the orifice are followed by the sap. 

 And thus it continues alternately to ebb and flow, with the changes of the 

 atmosphere, until the leaves are developed, when the external flow ceases alto- 

 gether, since this superabundance of sap is evaporated by the leaves. Now, 

 nature is ready to begin the true function of vegetable life. 



The roots, or, speaking more definitely, tlie mouths, which are at or near 

 the extremities of the roots, are constantly at work, sucking in and forcing up 

 water and whatever mineral and vegetable matter the water may hold in 

 solution, or at least so much of it as that particular plant may require for its 

 growth and development, for vegetation has the power of selecting what is 

 adapted to its individual wants, and rejecting all else. 



This is called the crude sap, and is drawn in and forced up the body of 

 the tree into the leaves. The amount of water thus pumped up is immense. 



Hales, who first experimented in this matter, found there was suilicient up- 

 ward pressure from the stump of a grape vine in the spring, to balance a col- 

 umn of water thirty-six and a half feet high. The sap ascends the tree 

 through the wood, every part of it except the heart, which is, so to speak, 

 dead. When ib has reached the leaves it undergoes a complete change, by 

 contact with the air, giving off its surplus water by evaporation. 



The quantity of water given off in this way is truly wonderful. It is said 

 that a common sun-flower, only three feet high and containing between five 

 and six thousand square inches of leaf surface, has been found to give oif from 

 one to two pints of water in twelve hours ; this will give one some idea of the 

 vast amount of evaporation carried on by the leaves of our forest trees. This 

 process of evaporation seems to be the particular function of the under side of 

 the leaves, and here, as elsewhere, we see that wonderful adaptation of means 

 to the end, that characterizes all the work of our Creator. The cells of the 

 under side of the leaf are of a different form and much more loosely put to- 

 gether tlian those of the upper side of the same leaf, thus giving more and 

 larger intercellular spaces on the former than on the latter, and all these spaces 

 are filled with air. Take, for example, the leaf of the white lily. On the 

 upper surface there are only three thousand stomata to the square inch, while 

 on the under side of the same leaf, sixty thousand are found, As eva]ioration 

 takes place most rapidly in bright sunny weather, if, by chance the leaf should 

 be reversed and all these months exposed to the scorching rays of the sun, the 

 water would be given off so rapidly that it would be nearly or quite impossible 

 for the roots to supply the demand. 



"Let no presuming, impious railcr tax 

 Creative "Wisdom, as if aui^ht was formed 

 In vain, or not for desirable ends." 



The sap being condensed, having lost a large per cent of its water and gained 

 in density and richness, is now prei)ared to nourisli the plant and begins its 

 downward course. In cxogens, or outside growers, this movement is along the 



