GROWTH 
123 
size according to circumstances. This is illustrated by the 
petals of flowers (such for example as the tulip) , that open 
and close several times before they drop off. This motion 
is caused by temporary fluctuations of growth of the upper 
and lower surfaces of the petals In a similar manner is 
explained the change of position of the leaflets of certain 
plants, such as clover, oxalis, bean, and others, at night 
or in cloudy weather, and the more rapid motion of the 
leaves of the "sensitive" and other plants (Figs. 81, 82, 
and 96). 
FIG. 83. Structure of seeds. Bean (Phaseolus), pea (Pisum), castor oil 
\Jlicinus), lupine (LupinusY, Indian corn (Zea Mays). 
124. Growth and Nourishment. When a plant or 
plant organ is growing, the protoplasm is constantly 
forming new parts, and therefore must continually be 
renewed or nourished. The more rapidly new parts are 
formed, the greater the need for food. This need is pro- 
