SEED-GERMINATION AND SEEDLING-GROWTH. 15 



easily to the surface. In this class of plants the hypocotyl 

 lengthens but little and does not lift up the cotyledons. 

 Hence this class of plants, including those with one coty- 

 ledon, such as corn, can be planted five or six inches deep 

 and yet reach the surface in favorable soils. But, as a 

 rule, horticultural seeds should not be planted deeper than 

 is required to secure the needed supply of moisture. As 

 an example, it is often recommended to plant the sweet- 

 pea deep in trenches, gradually filling up to a depth of five 

 or six inches. But experience shows that if planted one 

 inch deep they will succeed much better when planted on 

 heavy soils. 



17. How Seedling-roots Grow. The tree-seedling per- 

 mitted to stand where the seed was planted extends the 

 main or water-feeding roots downward, if the soil is favor- 

 able, to a depth of several feet. The writer has traced the 

 tap-root of a Buckeye seedling one foot in height to a 

 depth of five feet. In the deep gullies or washes in the 

 loess soils of western Iowa we have seen roots of older trees 

 that were two inches in diameter at a depth below the tree- 

 crown of twenty feet. But the depth reached depends on 

 the nature of the subsoil and the depth of the water-level. 

 The branch or surface-feeding roots also reach farther 

 than is usually suspected. As a rule, if conditions permit, 

 the surface nitrogen-feeding roots extend beyond the 

 spread of branches. The deep extension of the seedling- 

 root that goes down where the seed is planted, and the 

 speedy extension of surface-feeding roots, has led 

 planters of groves and plantations of forest-trees in 

 Europe and America to plant seeds where the trees are 

 to stand. 



In practice it has been found .that seedlings not trans- 

 planted are much larger and thriftier when ten years old 

 than the two- or three-year-old plants set out when the 



