TBB LEAF 



75 



114. At the junction with tho hlade and at the base, 

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

 special structnve by which more or less rapid movements 

 are secured when the blade is stimulated through cmi- 

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

 These portions, marked off from the rest of the p>etiole 

 and often somewhat swollen, are called pulrini (singular, 

 pulvlnus). Thej' are well seen in the IJean and other 

 plants of the same faniil}'. 



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. Kach has a short 

 stalk, or petiolule. WJiun day- 

 light fails, tlie petiolules bend 

 more or less sliarpl}' 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 jjosition 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, tiic inner faces 

 ajipn'oaching each other more or less closely. 



117. It must not be suppjosed that the lowering of 

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

 part of the plant ; although Linnteus gave the name 



t. The. " .sleep " of the White 

 Lupiue. 



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 haliitnal or effected only in response to change of illu- 

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

 plants ? 



