THE AMERICAN BOTANIST 



65 



number them the world over. In order for the slowly changing 

 trees and shrubs to produce so many species, a long period of 

 time must have been necessary, and this suggests that the tree 

 groups are much older than those containing only herbs. The 

 fact that trees change more slowly than herbs is reflected in 

 the catalogue of every nurseryman, for there are always a 

 larger number of varieties catalogued for herbs than for woody 

 plants. 



Cause of Leaf Movement. — A large number of plants 

 are able to make changes in the position of their leaves as cir- 

 cumstances warrant. In many cases these changes seem di- 

 rected by something akin to^ intelligence, though explained in 

 other ways. One of the commonest instances of this is found 

 in the sunflowers, whose leaves, and often the flowers, turn 

 toward the sun all day. In the so-called "sleep" of plants the 

 leaves or leaflets usually fold together though the plants do not 

 sleep in the accepted sense of that term. Still another set of 

 movements seem connected with evaporation. On a hot, dry 

 day, the leaves of corn roll up, the compass plant sets its great 

 leaves on edge and the leaves of many other plants, especially 

 those of the Leguminosae, assume positions that ensure a re- 

 duced evaporating surface. An investigation of such phenom- 

 ena has shown that such changes are entirely automatic and 

 are produced by the very condition which the leaves seek to 

 avoid. When one side of a thin board begins to dry out, the 

 fibers contract and in consequence it warps or curves toward 

 the dry side. In a similar way leaves may lose enough water 

 from the upper surface to cause the whole leaf to roll up. In 

 cases where the leaf changes position without rolling, some 

 special part of the petiole usually reacts to evaporation. 

 Changes of this kind may be produced artificially by the appli- 

 cation of some drying agent, like alcohol, to the surface of the 

 leaf. 



