268 LEAVES 



summer drought and thereby decrease their transpiring surface. 

 Of course this is not a protection to the leaves but to the plant. 

 Most trees of the temperate region shed all their leaves in autumn. 

 Such trees are known as deciduous. This shedding of leaves in 

 autumn protects the plant against transpiration during winter. 

 Even with leaves absent, trees are sometimes killed by trans- 

 piration from buds and twigs. The killing by transpiration in 

 winter is not due to a great water loss, but to the inability of the 

 roots to furnish water to compensate for the loss. Since the roots 

 of most trees are not far below the surface, a deep freeze may 

 freeze the water about them. Even when the soU is cold, roots 

 take up water slowly, and when the water is frozen into ice, they 

 can not absorb it at all. With only a little water furnished by 

 the roots, a small amount of transpiration may be sufficient to 

 cause the death of the cells in the buds and twigs. 



In transplanting trees, it is usually necessary to prune the top, 

 because the root system has been partly broken and cut away, 

 and consequently is not able to furnish enough water to compen- 

 sate for the amount transpired from a shoot of normal size. 

 Pruning the top results in fewer leaves and hence less transpiring 

 surface. Even after trees have been transplanted and well 

 established, a reduction of the transpiring surface by pruning the 

 top is often helpful, but usually the pruning of such trees has 

 other purposes as pointed out in the study of buds. However, 

 since the leaves are food-making organs, only a limited number 

 of them can be removed or the plant will suffer from starvation. 



Supplying moisture to the soil protects against injuries result- 

 ing from transpiration. Plants in the greenhouse must have the 

 soil about their roots kept moist by watering. In the dry 

 western regions water is supplied to the soil by methods of 

 irrigation. 



Much can be done in protecting against transpiration by 

 conserving the moisture of the soil. If an orchard is in sod, 

 many tons of water will be lost from the soil through the trans- 

 piration of the grass. By plowing and keeping the ground free 

 from grass and weeds, the water of the soil is conserved for the 

 fruit trees. In regions where dry farming is practiced, the 

 ground is fallowed during one year and then seeded the second 

 year. Fallowing consists in keeping the ground plowed and well 

 harrowed, so that the surface will be covered with a mulch and 



