326 GENERAL SCIENCE" 



tissue and is called the cambium layer. All the growth 

 of our hardy trees, such as the oak and maple, takes place 

 from the cambium layer. It forms new wood on the 

 inside and coarse bark on the outside. The outside bark 

 of a tree is dead and serves only to protect the tree in 

 various ways. 



Leaves. As soon as a seed germinates, leaves are 

 formed on its stem. Some plants, such as the bean and 

 the squash, have two seed leaves which appear as soon 

 as the seed has swelled in the process of germination. 

 Leaves are essential to plant life, as may be proved by 

 removing the leaves from a young plant. 



The broad part of a leaf is called the blade ; the stem is 

 called the petiole. Leaves may be classified according 

 to the arrangement of the veins (Figure 288). Number 

 1 shows the palmate-veined leaf of the maple ; number 

 2, the parallel- veined leaf of the wild lily-of -the- valley ; 

 number 3, the pinnately-veined leaf of the birch ; 

 number 4, the pinnately compound leaf of the rose. 

 Veins in leaves serve the double purpose of supporting 

 the parts of the leaf and of furnishing tubes to carry food. 



In almost every case the arrangement of the leaves 

 on a plant is such as to secure the greatest amount of 

 sunlight for them. In dense forests the trees grow tall 

 with clusters of leaves near the top, while in open fields 

 the same species of tree sends out numerous lateral 

 branches covered with leaves. On the smaller plants 

 the leaves arrange themselves in various ways, but 

 always so that each leaf receives a large amount of light. 



The cells of a leaf are composed of protoplasm and a 

 green material called chlorophyll. The layer of cells 

 forming the upper and lower part of the leaf is called the 

 epidermis. The epidermis on the upper part of the leaf 



