3 A TEXT-BOOK OF BOTANY 
such a plant grows in the shade the leaves do not assume 
the profile position. It must not be supposed that there 
of is any accuracy in the 
es B a north or south direction, 
Ve oe, as the edgewise position 
is the significant one. In 
the rosinweed probably 
the north and south di- 
rection is the prevailing 
one; but in the prickly 
lettuce, a very common 
weed of waste grounds, 
and one of the most 
striking of the compass 
plants, the edgewise 
position is frequently as- 
sumed without any ref- 
erence to the north or 
| Mal ye south direction of the 
W | apex (Fig. 26). 
= ly (9) Motile leaves have 
Fic. 26.—Prickly Je(tuce, showing the edge- the power of shifting 
wise or profile leaves from two points of their positions according 
view. 
to their needs, directing 
their flat surfaces toward the light, or more or less inclining 
them. Such leaves have been developed most extensively 
in the great family to which peas and beans belong, the 
most conspicuous ones being those of the so-called sensitive 
plants. The name has been given because the leaves respond 
to various external influences by changing position with 
remarkable rapidity. A slight touch, or even jarring, will 
call forth a response from the leaves; ancl the sudden ap- 
plication of heat gives striking results (ig. 27). The most 
common sensitive plant abounds in dry regions, and may 
be taken as a type of such plants. The leaves are divided 
