338 



The Living Plant 



the place of most active growth in each part, whether at tip, base, 

 or all through the structure. This, however, is easily determined 

 in another way, viz., by marking the parts when young by evenly- 

 spaced lines, the spread of which, as the parts grow up, must 



reveal the place where these 

 grow the most. If a young 

 root be thus marked by cross 

 lines, the result is like that of 

 our figure (figure 126). Evi- 

 dently young roots grow almost 

 wholly at their tips. If stems 

 be marked in the same way, the 

 result is somewhat different 

 (figure 127). Evidently young 

 stems grow mostly at their 

 tips, but over a much larger 

 area than the roots, as indeed 

 one might infer from the way in 

 which the nodes of young stems 

 spread apart. It is no trouble at all to find an adaptive reason for 

 the difference in the mode of growth of roots and stems, when one 

 recalls that roots must pick their way through the irregular in- 

 terstices of a closely-pressing soil, while stems have all outdoors 

 to expand in. As to leaves, their shape makes it necessary to 

 mark them by cross lines, forming squares, and when thus treated 

 the spread of the lines shows that leaves, unlike roots and stems, 

 grow all through their structure (figure 128). Slender leaves, 

 however, especially the kind that grow up from bulbs, grow al- 

 most wholly at the base. 



Although growth is typically accompanied by increase in 

 length, it sometimes is correlated with shortening. One case 

 thereof is found where a straight structure becomes a spiral, 

 as in tendrils, which thus pull their plants closer to a support, 

 or in the peduncles of some water plants, which thus draw their 



FIG. 126. A young Bean root, \ 

 just marked by evenly spaced 

 cross marks, and the same root 

 a day later. 



