THE LEAF 



75 



114. At the junction with the blade and at the base, 

 next to the stem, portions of the petiole may possess a 

 special structure by which more or less rapid movements 

 are secured when the blade is stimulated through con- 

 tact or injury or by changes in the intensity of light. 

 These portions, marked off from the rest of the petiole 

 and often somewhat swollen, are called pulvini (singular, 

 pulvinus). They are well seen in the Bean and other 

 plants of the same family. 



115. Of periodic movements executed by the action of 

 the petiole, the '' sleep " move- 

 ments of numerous plants are to 

 be noted. Figure 59 represents 

 the leaflets of the White Lupine 

 at night. The blade is here 

 divided into five or more parts, 

 or leaflets. Each has a short 

 stalk, or petiolule. When day- 

 light fails, tlie petiolules bend 

 more or less sharply downward. 

 When this action is most vigor- 

 ous, as in some of the younger 

 leaves, the leaflets are brought 

 closely together ; and they are 

 retained in this position with some force. With the return 

 of daylight the petiolules are stimulated to elevate the 

 leaflets again. ^ 



116. When the cotyledons of seedlings exhibit sleep 

 movements, they usually fold upward, the inner faces 

 approaching each other more or less closely. 



117. It must not be supposed that the lowering of 

 leaves or leaflets in such cases is an act of resting on the 

 part of the plant ; although Linnaeus gave the name 



5<). The 



sleep " of the White 

 Lupine. 



1 Try the effect of keeping seedlings of Clover, Oxalis, Bean, or 

 Lupine in the dark until late in the forenoon, or even all day. Are the 

 sleep movements habitual or effected only in response to change of illu- 

 mination ? Is lamp light or electric light bright enough to wake sleeping 

 plants ? 



