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, oxaHs, 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. Sj. — Structure of seeds. Bean (Pliaseolus), pea (Fisum), castor- 

 oil {Riciniis), lupine (Lupinus), Indian com (Zea Mays). (Photo by the 

 author.) 



124. Growth and Nourishment. — ^^'hen 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- 



