COMPASS PLANTS 



139 



dow through which a strong light is received. Continue the 

 motion of the clinostat until the leaves have attained average 

 size, for which several days 

 will be necessary, and note 

 the position of the leaflets. 

 Now place a second plant on 

 the clinostat in the same man- 

 ner, but set the apparatus to 

 direct the axis of the plant 

 parallel to the window and 

 note the final positions of the 

 leaflets. 



212. Compass Plants. 

 Leaves exposed to sunlight 

 in a horizontal position receive 

 the rays of a noonday sun 



FlG. 70. Arisaema triphyllum rotated 

 on horizontal axis parallel to window. 



at right angles, and are thus subjected to the mapdmum intensity 

 of illumination. Large numbers of species avoid such intensities 

 by growing only in shaded habitats. A few forms attain a similar 

 end by placing the leaves with the edges vertical and directed 

 north and south, on which account they have become known as 

 " compass plants." A leaf in this position receives two maxima 

 of illumination daily, one in mid-forenoon and one in mid-after- 

 noon, but these maxima are far below the maximum to which a 

 horizontal leaf is exposed. 



Grow a number of Lactuca in the open air where they may 

 receive sunlight during the entire day and note the position of 

 the leaves. Grow a similar number under the shade of a tree 

 or a building, and compare the results. Note the torsions nec- 

 essary to carry the leaves to their positions. Other compass 

 plants are Silphium laciniatuni and Wyethia. 



213. Other Reactions due to Intensity of Illumination. The 

 varying intensity of light has been the cause to which may be 

 ascribed several adaptations on the part of the plant by which 

 the injurious exposure may be avoided. Chief among these are 



