. 
182 PRACTICAL COURSE IN BOTANY 
interesting example of this kind of adjustment is furnished 
by the rosinweed, or compass plant (Silphium laciniatum, 
Figs. 238, 239), which grows in the prairies of Alabama and 
westward, where it is exposed to intense sunlight. The 
leaves not only stand vertical, but have a tendency to turn 
their edges north and south so that the blades are exposed 
only to the gentler morning and evening rays. The prickly 
lettuce manifests the same habit in a less marked degree. 
200. Night and day adjustments. These are move- 
ments in response to changes in the degree of illumination 
and temperature, as evidenced by the fact that they become 
feeble and soon cease altogether if the plant is kept a suffi- 
cient time under uniform conditions as to these two factors. 
(Exp. 74.) They are called “ nyctitropic ” or sleep move- 
ments, because they are most obvious in certain plants that 
undergo periodic adjustments to the alternations of day and 
might suggestive of an imaginary likeness to the sleep of ani- 
é — mals. Examples are 
oF most frequently met 
with among members of 
the pea family (Legumi- 
nose), the spurges 
(Euphorbiacee), and the 
sorrel (Oxalis) family. 
They are found among 
other species also, and 
indeed are much more 
240 241 
Fias. 240, 241.—A plant of the guayule general than is usually 
(Parthenium argentatum), to the leaves of which 
indexes have been affixed to show their day and supposed, ‘ most plants 
night position: 240, day position; 241, night Showing signs of them 
tas (From photographs by Prof. F. E. if carefully tested. A 
simple way of doing this 
is by attaching bristles about two inches long to the tips of 
two leaves on opposite sides of the stem, as in Figs. 240, 241, 
and comparing the divergence of the bristles during the day 
and at nightfall. In this way a change of position in the 
