THE LIFE OF THE TREES 5 



stipules are conspicuous. The black willow has large, 

 leaf-like, heart-shaped stipules, green as the leaf and saw- 

 toothed. 



Most stipules shield the tender leaf during the hours of 

 its helplessness, and fall away as the leaf matures. Others 

 persist, as is often seen in the black willows. 



With this second vernal leaf fall (for stipules are leaves) 

 the leaves assume independence, and take up their serious 

 work. They are ready to make the living for the whole 

 tree. Nothing contributed by soil or atmosphere — no 

 matter how rich it is — can become available for the tree's 

 use until the leaves receive and prepare it. 



Every leaf that spreads its green blade to the sun is a 

 laboratory, devoted to the manufacture of starch. It is, 

 in fact, an outward extension of the living cambium, 

 thrust out beyond the thick, hampering bark, and special- 

 ized to do its specific work rapidly and effectively. 



The structure of the leaves must be studied with a 

 microscope. This laboratory has a delicate, transparent, 

 enclosing wall, with doors, called stomates, scattered over 

 the lower surface. The "leaf pulp" is inside, so is the 

 framework of ribs and veins, that not only supports the 

 soft tissues but furnishes the vascular system by which an 

 incoming and outgoing current of sap is kept in constant 

 circulation. In the upper half of the leaf, facing the sun, 

 the pulp is in "palisade cells," regular, oblong, crowded 

 together, and perpendicular to the flat surface. There are 

 sometimes more than one layer of these cells. 



In the lower half of the leaf's thickness, between the pal- 

 isade cells and the under surface, the tissue is spongy. 

 There is no crowding of cells here. They are irregularly 

 spherical, and cohere loosely, being separated by ample 



