LIGHT AND VEGETATION. 201 



tion, or the power of assuming this position by motor or impassive 

 wilting movements. 



Among the plants of the temperate zone the so-called compass 

 plants are examples of similar adaptations. The compass plants in- 

 clude, among others, the wild lettuce {Lactuca scariola) and rosin 

 weed (Silphium laciniatum). These plants place the leaves in a 

 vertical position with the tips pointing north and south in such man- 

 ner that the direct rays of the morning and evening sun only may 

 strike the surfaces at right angles, while the edges are presented to 

 the fierce rays at noonday. That this arrangement is an adaptation 

 against the intense light is evident when it is seen that specimens 

 growing in shaded locations or in diffuse light place the leaves in 

 the typical horizontal position. To meet the functional conditions, 

 both sides of the compass leaves are almost equally provided with 

 palisade cells for food formation and stomata for transpiration. The 

 estimation of the light striking a compass leaf shows that it receives 

 approximately the same amount of light as a horizontal leaf during 

 the course of a day, but the two maxima of intensity, morning and 

 evening, are much below that of the noon of horizontal leaves. 



The influence of light upon plants may be briefly summed as 

 follows : 



Light is necessary for the formation of food substances by green 

 plants, and it is an important factor in distribution in land and ma- 

 rine forms. 



Growth and reproduction are generally retarded by the action of 

 the blue-violet rays. 



Light is fatal to certain bacteria and other low forms of vegeta- 

 ble life. 



Many plants have the power of accelerated growth of stems in 

 diminished light as an adaptation for lifting the leaves above " shad- 

 ing " objects. 



The growth of many leaves and of the perianth of flowers is 

 hindered in diminished light. 



The outward form of many organs, particularly leaves, is de- 

 pendent upon the intensity of the light received. 



The internal structure of bilateral or dorsiventral organs is large- 

 ly determined by the direction of the rays. 



Plants have the power of movement to adjust their surfaces to a 

 proper angle with impinging light rays, as a protective adaptation. 



Matches which do not contain any phosphorus and which take fire by 

 friction on any surface — a match that has been long sought — have been 

 prepared by Mr. S. A. Rosenthal and Dr. S. J. von Kornocki. It is repre- 

 sented that they can be manufactured as cheaply as ordinary matches. 



