80 



PHYSIOLOGY. 



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

 creases until it reaches the maximum. Then the rapidity of elongation les- 

 sens as the cells come to lie farther from the tip. The period of maximum 

 elongation here is the zone of maximum growth of these cells. 



192. Just as the cells exhibit a zone of maximum 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 diminishes in rapidity again. So 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. 



193. 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 first leaves are comparatively 

 small. As the plant grows larger the 

 leaves are larger, and this increase in 

 size 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- 

 mum size of the leaves on the stem. The 

 rapidity and energy of growth of the stem 

 i- also correlated with that of the leaves, 



and the zone of maximum 

 growth is coincident with 

 that of the leaves. It would 

 be instructive to note it 

 in the case of other plants 

 and also in the case of 

 fruits. 



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

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

 at a given moment on the south side are elongating more rapidly than the 

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

 north. In a few moments later the cells on the west side are elongating more 

 rapidly, and the stem is turned to the east; and so on, groups of cells in suc- 

 cession around the stem elongate more rapidly 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. 67. 

 Lever auxanometer (Oels) for measuring elongation of 

 tlie stem during growth. 



