GROWTH 
119 
an advantage to the plant, since 
the leaves are thereby more 
rapidly brought into positions 
of best exposure to air and sun- 
light. The growth of several 
internodes at the same time, 
and their elongation throughout 
their entire length, carries the 
tip of the stem forward with 
much greater force than if 
growth were confined to a short 
zone, as in the case of the grow- 
ing root. But a more rapid and 
forceful advance of the root-tip 
through the soil might result in 
serious or fatal injury, on ac- 
count of the resistance and ob- 
stacles encountered in the soil. 
Thus the different manner of 
growth of stems and roots is seen 
to be of direct advantage to the 
plant as a whole. 
122. Growth of Leaves. In 
tropical climates leaves that 
have once begun to form con- 
tinue to grow until they reach 
full size; but in temperate cli- 
mates, having an alteration of 
summer and winter, this is not 
the case. Here the leaves of any 
given season are all formed dur- 
ing the preceding growing sea- 
son, and remain over winter 
FIG. 78. Diagram showing 
mode of growth in length of a 
portion of a stem of bindweed 
(Convolvulus} . A , stem with in- 
ternodes marked off into inter- 
vals of i cm.; B, the same stem 
24 hours later, showing the rela- 
tive elongation of the various 
internodes. (Cf. Fig. 77.) (Re- 
drawn from Bonnier and Leclerc 
du Sablon.) 
