66 



Structure of Leaves and of Wood. 



as the sap descends under the bark it is much less watery, and is 

 charged with the materials that by assimilation may form the new 

 layer of wood, and every growing part of the whole structure. 



238. The materials thus won from the earth and the air, are added 

 to the tree, as it gains in size, or dropped to the earth with the fall- 

 ing leaves and the fruit the leaves to add fertility to the soil, and 

 the seeds of the fruit to furnish germs for new creations of the 

 parent type. 



239. The form and size of leaves may vary at different ages of the 

 tree and upon different parts of the same tree at the same time. Upon 

 thrifty young sprouts they are generally larger than upon the old 

 branches. As a rule they become smaller at great elevations. In the 

 eucalyptus, the young trees bear heart-shaped and horizontal leaves, 

 and the old trees sickle-formed leaves that stand in a vertical plane, ex- 

 posing both sides to the light, and shading the ground but little. The 

 upper leaves upon the holly are less notched and less spiny, when the 

 tree becomes old. In certain of the cedars, cypresses, and other con- 

 ifers with imbricating leaves, the scales may at certain stages of growth 

 elongate into linear leaves, very different from the more usual form. 

 The same tree may present both forms at the same time, but the linear 

 leaves are more common upon young trees than upon old ones. In 

 certain forms of disease, the linear form is sometimes assumed by 

 these imbricating leaves. 



Structure of the Wood and Bark. 



240. The trunk of an exogenous tree shows three distinct kinds 

 of structure. In the twig of oak of one year's growth, we find the 



inner part is filled with the pith, m, 

 m an extremely light cellular body 

 which appears essential to new 

 K growth, and is always present in a 

 e twig covered with leaves; but in 

 the trunk of the tree it becomes 

 nearly or quite obliterated, and in 

 fact may perish altogether without 

 apparently affecting the growth of 

 the tree. The pith or medulla is 



50. Cross-secti^IfanOakTwigofone surrounded by a sheath of hard 



cellular tissue, and outside of this 



Year's Growth.(i) 



1 Thi? nnd tlio two following figures are from Rossmassler's work "Der 



