12 



INTRODUCTION TO BOTANY 



stem will be discussed in later chapters. By means of their 

 upright position, stems may support the branches and leaves 

 in such a way that they are upheld in the air. Support is evi- 

 dently one of the conspicuous functions served by plant stems. 

 Many plants live for only one year (annuals) or two years 

 (biennials) ; in such plants large and strong stems are not 

 often found. Other kinds of plants may live for two or more 

 years (perennials'). Woody perennials may live for hundreds 

 of years, and their stems usually 

 increase in thickness from year 

 to year, until, in the case, of trees, 

 stems are sometimes developed 

 which are several feet in thick- 

 ness, much over a hundred feet 

 in height, and of such strength 

 that very heavy tops are upheld 

 throughout the severest storms. 

 The increase in thickness is shown 

 by the annual rings of wood (fig. 8). 

 Often the age of a woody stem 

 may be indicated approximately 

 by its annual rings, but that these 

 are not always truly annual rings 

 is shown by the fact that when 

 there are intermittent favorable 

 and unfavorable growing periods within the same season, 

 more than one ring may be formed within one year. Cases 

 are known of young trees that have almost twice as many 

 rings as the number of years of their age. 



10. Stems as passageways for liquids. When the stem of 

 an actively growing plant is cut, water sometimes exudes 

 upon the cut surface of the stump. The same outpouring of 

 water is seen when a leaf of a corn plant is peeled away from 

 the stalk. When we wish to keep cut flowers in a fresh con- 

 dition, we place their stems in water. If we should place a 

 fresh leaf of celery in a solution of one of the aniline dyes 



FIG. 8. One quarter of a cross 



section of a stick of oak wood 

 m, medullary rays, running from 

 bark to pith; r, annual rings; b, 

 boundaries between rings, porous 

 from presence of many ducts ; i, in- 

 terior fibrous layers of dead bark ; 

 pi, hard plates of dead bark, split- 

 ting away from each other but at- 

 tached to bark beneath. Reduced 



