GRO WTH. 



123 



elongated slowly at first. The rapidity of the elongation of these cells in 

 creases unlil it reaches the maximum. Tiien the rapidity of elongation les- 

 sens as the cells come to lie farther from the tip. Tlie period of ma.\imum 

 elongation here is the zone ef inaximum gro2^'lh of these cells. 



255. Just as the cells exhibit a zone of maxiraum growth, so the members of 

 the plant exhioit a similar zone of maximum growth. In the case of leaves, 

 when they are young the rapidity of growth is comparatively slow, then it 

 increases, and finally diminislies in rapidity again. S.j it is with the stem. 

 When the plant is young the growth is not so rapid; as it approaches middle 

 age the rapidity of growth increases; then it declines in rapidity at the close 

 of the season. 



256. Energy of growth. — Closely related to the zone of maximum growth is 

 what is termed the energy of growth. This is manifested in the compara- 

 tive size of the members of a given plant. 

 To take the sunflower for example, the 

 lower and fir=t leaves are comparatively 

 small. As the plant grows larger the 

 leaves are larger, and this increase in 

 -ize of the leaves increases up to a maxi- 

 mum period, when the size decreases 

 until we reach the small leaves at the top 

 of the stem. The zone of maximum growth 

 of the leaves corresponds with the maxi- 

 munt size of the leaves on the stem. The 

 rapidity and energj' of growth of the stem 

 is also correlated w-ith that of the leaves, 



and the zone of maximum 

 growth is coincident with 

 that of tlie leaves. It would 

 be instructive to note it 

 in the case o± other plants 

 and also in the case of 

 fruits. 



257. Nutation. — During the growth of the stem all of the cells of a given 

 sectiori of the stem do not elongate simultaneously. For example the cells 

 at a given moment on the south side are elongating more rapidl)' than the 

 cells on the other side. This will cause the stem to bend slightly to the 

 north. In a le\\' mijnients later the cells on the west side are elongating more 

 rapidly^ arjd the stem is turned to the east: and so on. groups of cells in suc- 

 cession around the stem elongate more rapidh' than the others. This causes 

 the stem to describe a circle or ellipse about a central point. Since the re- 

 gion of greatest elongation of the cells of the stem is gradually moving toward 

 the apex of the growing stem, this line of elongation of the cells which is 



Fig. III. 

 Lever auxanonieier '(Jcls) for measuring elongation of 

 \\\t stem during growtli. 



