Roots and Their Environment 185 



ated when one sees cement sidewalks broken and large rocks 

 moved by the growth of roots under them. Growth pres- 



FlG. 105. Fern leaves pushing upward through a cement sidewalk. 

 {After G. E. Stone.) 



sure is just as powerful in stems and other growing parts. 

 Fleshy roots like those of the radish and turnip sometimes 

 force themselves partly out of the ground by the thickening 

 of the upper portion. 



Root contraction. As roots mature, they may contract 

 in length and so draw the base of the stem a slight distance 

 into the soil. In this way crevice plants on cliffs are con- 

 tinually held firmly in place, in spite of the wearing away of 

 the cliff face by erosion. In the same way the crowns of 

 clover and plantain roots that have been lifted up by frosts 

 may be drawn into the soil, and small bulbs and tubers, many 

 of which are formed at higher levels than the parent bulbs, 

 may be pulled deeper into the soil by root contraction. 



Root duration. The roots of various plants are annual, 

 biennial, or perennial. Perennial plants may have either 

 annual or perennial roots, just as they may have either an- 

 nual or perennial aerial stems. Plants with bulbs, tubers, or 

 corms grow a new set of roots each year. Plants with root- 

 stocks, like the May apple and Solomon's seal, generally 

 have roots that last for several years. Shrubs and trees also 

 have perennial roots. We must be sure to understand, how- 



