52 THE STEM 



oped even in annual stems. If the plant is a perennial, 

 especially if it grows to a considerable height, the wood 

 increases and the living tissue becomes a rehitively smaller 

 part of the whole. In the stems of trees the living por- 

 tions comprise only the growing tips of branches, the 

 younger bark, and a film of active tissue just outside the 

 wood. The bark (except those parts freshly formed), and 

 the cylinders of wood, are essentially dead, and serve 

 merely mechanical purposes in the support and protection 

 of that which is alive. 



71. The growth of stems. — Stems increase iu length at 

 or near the young tips. In plants of defiJiite annual 

 growth the number of internodes — or interspaces between 

 leaves — is predetermined in the bud. Early in the fol- 

 lowing season these internodes gain their full extension 

 and thereafter remain fixed in length. Girth increases 

 through the formation of wood by the living tissue that sur- 

 rounds the woody cylinder. Growth is, of course, inter- 

 rupted as often as severe cold or extreme drought sets in ; 

 and in those parts of the world where this is a regularly 

 recurring event, the wood is formed in successive layers. 

 When cut across, the layers appear as rings. Stems of trees 

 and shrubs grown in temperate climates show in the cross 

 section the spi'ing wood — laid down when growth is par- 

 ticularly active — differing in color or texture from the 

 fall wood. The age of trees, therefore, is easily made out 

 when the trunk is cut off. Sometimes, however, two 

 rings are formed in a single season, when midsummer 

 drought interrupts the regular growth. Allowance must 

 be made for these cases in estimating the age of trees. 



72. The direction of growth. — Most stems grow upward; 

 that is, toward the light ; for it is the benefit got by full 

 exposure of the foliage to the sun that has led to tall 

 stems. Leaves of tall-stemmed plants are raised out of 

 the shade cast by crowding neighbors. 



73. Upright stems include, besides the ordinary rigid 

 and self-sustaining type, many climbing forms. Certain 

 ones gain the advantages of elevation by twining upon the 



