52 THE STEil 



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

 especiall}- if it grows to a considerable height, the wood 

 increases and tlie living tissue becomes a relativelj' smaller 

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

 tions comprise only the gro\\dng tips of branches, the 

 younger bark, and a tilm 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 wliicli is alive. 



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

 or near the young tips. In plants of definite annual 

 growth the number of interuodes — or interspaces between 

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

 lowing season these interuodes gain their full extension 

 and thereafter remain fixed in length. Girtli increases 

 through the formation of wood l)y the living tissue that sur- 

 rounds the woody c\linder. Growth is, of course, iuter- 

 riTpted as often as severe cold or extreme drouglit sets in ; 

 and in those parts of the world \\ here this is a regidarly 

 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 spring wood — laid down when growth is par- 

 ticularly active — differing in color or texture fi'om the 

 fall wood. The age of trees, tlierefore, 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 tlie age of trees. 



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

 that is, toward the light ; for it is the Ijenefit got liv fidl 

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

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

 the .shade cast by crowding neighbors. 



73. Upright stems include, Ijesides the ordinary rigid 

 and self-sustaining type, many climbing forms. Certain 

 ones gain the advantages of elevation by twiidjig upon the 



