GROWTH. 



157 



the 

 The 



217. Growing region.— The part of any one of 

 larger plants which is growing in length is limited, 

 elongating region of a root rarely 

 exceeds a centimeter, and is often 

 not more than one-half a centi- 

 meter in length. In stems, how- 

 ever, the elongating part may 

 measure twenty or even fifty cen- 

 timeters, and in rare cases much 

 more. Figure 119 shows a root, 

 A, upon whose surface marks were 

 made i mm. apart. Twenty-four 

 hours later the root presents the 

 appearance of B. Only the tis- 

 sues in the first five spaces were 

 capable of elongation. The 

 others had passed into the third 

 phase. The second and third 

 millimeters grew most in length. 

 The growing regions of stems may 

 be determined in the same way. 



EXERCISE XXXVIII. 



To determine the zone of maximum Fig. 119.— ^, a young root of the pea 

 growth in roots and stems. ™="'''=^ ™'* ^""^ '"''^ °' Chinese 



A. Arrange four seedlings as in 

 T[ 205, with roots vertical, in moist air. 



Which spaces grow most ? 



B. Mark several upper internodes 

 of ,a bean plant in a similar way, but 

 at 5 mm. intervals. After 48 hours observe how many have elongated 

 and which have grown most. 



218. Tension due to growth. — ^The different regions in 

 any organ usually do not grow at an equal pace, and con- 

 sequently certain parts are under strain, while others are 

 compressed. The curled and crinkled leaves or the curved 



ink into 13 spaces of i millimeter 

 each. B^ the same root, 24 hours 

 later, showing elongation only in 

 terminal 5 millimeters. The rate of 

 growth is greatest in the 2d and 3d 

 millimeters, and slow in the ist, 4th, 

 and 5th. Magnified 2 diam.— After 

 Frank. 



