252 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



2. Enlargement of Roots, Stems, and Leaves.- — Elongation in roots is confined to a 

 region near the tip. Different parts of the elongating region exhibit different rates of 

 enlargement; the most rapidly elongating portion is near the middle of the region, while 

 the parts above and below enlarge more slowly. The tip millimeter of a root of 

 Windsor bean elongated but 1.5 mm. in 24 hours, while the third millimeter from the 

 tip elongated 8.2 mm., and the tenth elongated only 0.1 mm. The tip millimeter of 

 another root of the same kind elongated 1.8 mm. during the first day, 17.5 mm. during 

 the fourth day, 7 mm. during the seventh day, and not at all during the eighth day. 



In stems, elongation is generally confined to a few internodes near the tip, and each 

 elongating internode exhibits different rates of enlargement for its different regions. 

 In onion leaves growth is confined to the basal portion in all but very young specimens. 



Growth may sometimes result in the contraction of an organ, as in the contractile 

 roots by which Crocus corms are pulled downward in the soil. 



3. Tissue Strains. — Since all parts of a plant organ do not enlarge at exactly cor- 

 responding rates, and since the various parts are all rather firmly joined, some tissues 

 become stretched and others compressed, by adjacent tissues. Strains may thus 

 be developed, in any direction, and they result in increased rigidity of the organ. 

 Each concentric layer of tissue in an elongating internode is stretched with respect 

 to the next layer within and compressed with respect to the next layer outside. The 

 bark of an enlarging willow shoot shows transverse stretching, while the inner part of 

 the shoot is compressed. 



