66 



General Botany 



Compass plants. There is another class of plants which are 

 sensitive to light, but which respond to it in a very different 



manner. These are the so-called 

 compass plants, of which the 

 wild prickly lettuce is a widely 

 distributed example. In sunny 

 situations the leaves of these 

 plants tend to take positions 

 edgewise to the direction of the 

 most intense light. As the sun- 

 light is most intense at noon, it is 

 only in the morning and late 

 afternoon that the flat sides of 

 the leaves are perpendicular to 

 the sun's rays. This response 

 to the hght also places most of 

 the leaves in a vertical north- 

 and-south plane and suggests the 

 name '' compass plant." When 

 grown in partial shade, the leaves 

 of these same plants are hori- 

 zontal. Hence it is clear that the 

 Fig. 36. Prickly lettuce, which is called positiou of their Icavcs in suuny 



;'compassplant''because its leaves stand situations is the result of hght 



in a north-south plane: A, viewed from ... 



west; B, viewed from south. Drawn COuditionS (Fig. 36). 



from a specimen grown under exposure to HoW the blade attains ItS pO- 



bright sunlight. 



sition with reference to the light. 



The position of the leaf blade is partly attained, as has been noted, 

 by the bending and twisting of the plant stem during its develop- 

 ment. To a much greater extent the blade owes its position to 

 the bending, twisting, and elongating of the petiole. Indeed, 

 its ability to place the leaf in an advantageous position toward 

 the hght is the particular advantage of the petiole. Its length 

 and direction of growth are for the most part determined by the 



