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PRACTICAL COURSE IN BOTANY 



interesting example of this kind of adjustment is furnished 

 by the rosinweed, or compass plant (Silphium lacinialum, 

 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 resj)onse 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 

 night suggestive of an imaginary likeness to the sleep of ani- 

 mals. Examples are 

 most frequently met 

 with among members of 

 the pea family {Legumi- 

 noscB), the spurges 

 ( Euphorbiacece) , and the 

 sorrel (Oxalis) family. 

 They are found among 

 other species also, and 

 indeed are much more 

 general than is usually 

 supposed, most plants 

 showing signs of them 

 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 



Figs. 240, 241. — A plant of the guayule 

 (Parthenium nrgentntuni) , to the loaves of which 

 indexes have been affixed to show their day and 

 night position: 240, day position; 241, night 

 position. {From, photographs by Prof. F. E. 

 Lloyd.) 



