PLANTING AND CARE OF TREES 



35 



Fig-. 17. Cross section through a Beech leaf, after Strasburger. ep, 

 ep", epidermis, the protective covering- of upper and lower surface of 

 leaf; pi, s and sp, cells containing- little granules of leaf-g-reen or 

 chlorophyll — the places where plant food is made; st, opening through 

 the epidermis connecting- the air passages in the leaf with the free air 

 outside. 



All parts of plants need oxygen. Eoots obtain it from the air 

 in the soil. If the roots are too deeply covered, as when the 

 ground is filled in above them in changing the grade, or if the air 

 in the soil is driven out by an excess of water, most trees are likely 

 to suffer. Some forms like the Elm, however, are naturally able to 

 endure such unfavorable conditions. 



The Leaf — It has been already said that trees are like ani- 

 mals in their general food requirements, but that they have the 

 added characteristic of being able to make out of elements obtained 

 from the soil and from the air the complex food which they require. 

 This manufacture of plant food takes place within the leaf in mi- 

 nute granules of leaf-green or chlorophyll (fig. 17). Sunlight is 

 necessary and furnishes the energy for the process. Carbonic acid 

 gas from the air enters through small openings into the leaf and is 

 there broken down into oxygen and carbon. The oxygen is re- 

 turned to the air in the pure state, while the carbon is united with 

 the water and minerals from the soil. The complex plant food 

 which is thus built up, is transported to all the growing parts of the 

 tree and is used in the construction of new leaves, roots and wood. 

 Carbon is the conspicuous element in wood, comprising one-half of 

 its dry weight, and in the production of charcoal is left in nearly a 

 pure state. When wood is comjiletely burned, the process ob- 

 served in the leaves is reversed. The water passes off as vapor, the 

 mineral matter is left in the fonn of ashes, while the carbon re- 

 unites with the oxA'gen of the air in the formation of cabonic acid 

 gas. The leaf may be compared to a factory. Carl)onic acid gas, 

 water and dissolved minerals are the raw materials. Plant food 



