28 HORTICULTURAL MANUAL. 



framework of the leaf gives strength and the ribs bring in 

 the ascending fluid and distribute it to every part by 

 means of the veinlets. The lower part of the leaf has its 

 air-chambers and breathing-pores, which vary in different 

 varieties and species less in structure than the upper sur- 

 face exposed to the sun. The upper surface exposed to 

 the sun is protected by a more compact structure, which 

 varies in differeot climates. If we examine under a micro- 

 scope the leaves of the Oleander, Duchess apple, or Con- 

 cord grape, we will find a greater part of the leaf's thick- 

 ness to be made up of elongated cells placed endwise close 

 together, so as to expose as little surface as possible that 

 is not protected. In interior climates the structure of the 

 leaves is quite a certain guide as to the ability of any variety 

 to endure the summer sun and periods of dry, hot air. 



The time of ripening of the leaf is also important as an 

 indication of hardiness in cold climates. Such hardy 

 fruit-trees as Duchess, Hibernal, and Yellow Transparent 

 apple, for instance, prior to the advent of cold weather 

 will show the leaf ripening, like those of the hard maple 

 and the oak, by change of color and the other changes 

 that precede natural dropping. This perfect ripening of 

 the leaf before cold weather is important also in the way 

 of storage of the cell-structure with reserve starch and 

 other food which ensures the perfection of the flowers and 

 the holding of the fruit the succeeding season. On the 

 other hand, the fruit-tree that has its leaves blackened by 

 the first severe freeze rarely has the reserve food stored for 

 safe wintering, or for the starting of new growth, flowers, 

 and fruit the next spring. 



31. Sap and its Movements. The movement of water 

 in cell-structure, transpiration in the leaf, and the down- 

 ward current of assimilated food belongs to botany. But 

 the work of the horticulturist soon shows the need of some 



