82 



PLANT LIFE. 



stems, however, the elongating part may measure twenty or even 



fifty centimeters, and in rare cases 

 much more. Figure 184 shows a 

 root, y4,upon whose surface marks 

 were made 1 mm. apart. Twenty- 

 four hours later the root presents 

 the appearance of B. Only the 

 tissues 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 deter- 

 mined in the same way. 



259. Tension of tissues. — The 

 different tissues in any organ usu- 

 ally do not grow at an equal pace, 

 and consequently certain tissues 

 are under strain, while others are 

 compressed. The curled and 

 crinkled leaves or the curved cap- 

 sules of mosses illustrate this in- 



Fig. 184. — A, a young root of the pea 

 marked with fine lines of Chinese equality. It may be present, how- 

 ink into 13 spaces of i millimeter ^ J J x ' 



each, b, the same root, 24 hours ever, without manifesting itself in 



later, showing elongation only in 7 <J 



terminal 5 millimeters The rate of external form. This general COll- 

 growth is greatest m the 2d and 3d ° 

 millimeters and slow in the ist 4 th, ditioil is kllOWll as the tension of 

 and 5th. Magnified 2 diam. — After J 



Frank - tissues. If the rapidly growing 



flower-stalk of the dandelion or the leaf-stalk of rhubarb 

 be carefully split lengthwise the parts will curve or even curl 

 outward. Separating the inner and outer tissues of a young 

 elder shoot and carefully measuring them shows that the 

 inner tissues elongate and the outer actually shorten. The 

 experiment, therefore, shows that the inner tissues really 

 grew more rapidly than the outer, but were compressed in 



