IRRITABILITY. 



I2 9 



were grown in a box, when they were about $$cm high were 

 covered for nearly two days, so that the light was excluded. 

 At midday on the second day the box was removed, and the 

 leaves on the covered plants are well represented by fig. up, which 

 was made from one of them. The leaves of the other plants 

 in the box which were not covered were horizontal, as shown 

 by fig. 120. Now on leaving these plants, which had exhibited 



Fig. 120. 



Sunflower plant removed from 

 darkness, leaves extending under 

 influence of light (diaheliotro- 

 pism.) 



induced "sleep" move- 

 ments, exposed to the light 

 they gradually assumed 

 the horizontal position again. 



265. Epinasty and hyponasty. During 

 the early stages of growth of many leaves, 

 as in the sunflower plant, the direction of 

 growth is different from what it is at a later 

 period. The under surface of the young 

 leaves grows more rapidly in a longitudinal 

 direction than the upper side, so that the 

 leaves are held upward close against the 

 bud at the end of the stem. This is termed 

 hyponasty, or the leaves are said to be 

 hyponaslic. Later the growth is more rapid 

 on the upper side and the leaves turn downward or away from the bud. 

 This is termed epinasty, or the leaves are said to be epinastic. This is shown 

 by the night position of the leaves, or in the induced "sleep "of the sun- 



Fig. II Q. 



Sunflower plant. Epinastic con- 

 dition of leaves induced during the 

 day in darkness. 



