Most plants under culture develop true leaves or what Gray terms ''Leaves as foliage." In horticulture the mode of connection of the leaf with the branch and root, and its relative thickness and texture, have more importance than its morphology and action as given by the botanist. Over a large part of the United States the texture and relative thickness and firmness of the leaf are far more important than in moister and cooler climates with less-continued clear air and sunshine. The 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 surface exposed to the sun. The upper surface exposed to the sun is protected by a more compact structure, which varies in different climates. If we examine under a microscope the leaves of the Oleander, Duchess apple, or Concord grape, we will find a greater part of the leaf's thickness 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.