STUDIES OF SEED-PLANTS 111 



and thereby forcing growth in other directions. For example, 

 pruning off a terminal bud makes the lateral branches grow. 

 Farmers' Bulletin No. 181 should be consulted, and a field-trip 

 taken to some well-maspaged orchard or tree-nursery. 



Nature's method of pruning by crowding out superfluous 

 branches is well illustrated in any dense woods. The shaded 

 branches soon die and decay, and the result is the growth of 

 long straight stems with living branches near the top only. 

 Why are trees set close together in young forest plantations ? 



Pruning trees and shrubs at planting time is of great value, 

 in that it reduces the number of leaves until roots can get 

 well established in the soil. Unless the leaf-surface is thus 

 reduced, evaporation is likely to be greater than can be re- 

 placed by absorption of water by the roots. It is difficult 

 to transplant evergreens, because pruning would spoil the 

 form of these ornamental trees, and the leaves cause excessive 

 evaporation before new roots are established in the soil. 



168. Duration of Life of Plant Stems. It is a matter of 

 common observation that some plants live only one year and 

 that others live many years. All plants of which the life of 

 a generation, beginning with the seed and ending with death 

 of stem and roots, comes within a year are called annuals. 

 That is to say, an annual plant is one which completes its life- 

 history by starting from seed, producing seed and dying, root 

 and all, within a single year. Some plants (e.g., beet, cab- 

 bage, thistle, mullein) start from seed one year, the roots 

 and base of the stem live over the winter, and in the second 

 summer they blossom, produce seed, and then die. Such two- 

 year plants are called biennials. Still other plants (especially 

 trees and shrubs) live more than two years ; and such plants 

 are perennials. Some perennials have a soft stem which can- 

 not withstand the winter, but the underground parts are 

 hardy and new stems form above ground in each growing 

 season. Examples are peony, golden-rod, hollyhock, as- 

 paragus, violet, dandelion, buttercup, and most common 



