22 HORTICULTURAL MANUAL. 



24. Heartwood and Sapwood. In the one-year-old seed- 

 ling the stem is composed wholly of live or sap wood. But 

 with increased age the older layers or rings of growth are 

 buried by the newer ones. The newer layers with light 

 color on the outside are called sapwood or alburnum. As 

 the tree gets old the inner wood becomes drier, darker, 

 and more solid. This interior darker wood is not alive. 

 If kept from the air by the live wood and perfect bark it 

 may remain sound for a century or more, but if the air is 

 admitted by cutting or accident it will soon make a rotten 

 spot in the stem. Such rotten spots often result in the 

 interior States from sunscald of the stem on the south side. 

 Premature darkening of the wood or " black heart " may 

 also result from feeble growth from any cause after trans- 

 planting. But it more frequently comes in the prairie 

 States from severe winter freezing. This winter injury 

 often extends to near the cambium layer, and the tree may 

 survive feebly at first, but soon the injury is covered by 

 new growth. But in such cases the heart wood never 

 assumes its proper color. 



25. Proper Height of Fruit-tree Stems. The advice in 

 the past has been to trim up nursery trees when planted 

 in orchard high enough to work under the branches. At 

 this time in all parts of the Union -even in California and 

 the South the advice of experienced orchardists is in 

 favor of stems of orchard trees not to exceed three feet in 

 height. During the first year's growth in orchard the 

 exposed stem four or five feet in height is apt to be so 

 injured on the south side that growth is checked with 

 consequent check to growth of roots. The increased 

 growth on the north side of the stem, as well as branches 

 and root, soon causes the tree to lean to the north with 

 increased liability to stem injury. Even isolated shade- 

 mid park-trees in the prairie States, when the stems are 



