74 EXPERIMENTAL PLANT PHYSIOLOGY. 



ture, and leaves which have been artificially twisted, so that 

 the lower side is exposed to the light, reverse the arrangement 

 of these two kinds of parenchyma. 



EXPERIMENT 94. 



INFLUENCE OF LIGHT ON THE STRUCTURE OF LEAVES. 



Turn and fasten young leaves of the Beech (Fagus sylvatica) so 

 that the under side is exposed to the light. When mature they 

 show palisade tissue in the side now above, and spongy parenchyma 

 in the side turned away from the light, as may be seen on examina- 

 tion with the microscope. 



EXPERIMENT 95. 



DEVELOPMENT OF FLOWERS IN DARKNESS. 



Enclose a young inflorescence of Scarlet Runner or Morning- 

 glory in a pasteboard box or bag of thick black cloth. The flowers 

 and fruit will develop normally in the darkness thus secured. 



56. Influence of Gravity and Light on the Formation of Organs. 

 Light and gravity influence the origin and demarkation of 

 the forms of organs in a very remarkable manner. If a twig 

 of Willow or some other plant is placed in a damp chamber, 

 root and leaf buds will develop under the bark. If the twig is 

 placed in an upright position, the roots will develop below and 

 the leaves above. This " polarity " is, according to Vochting, 

 due to light and gravity. The action of light induces the 

 formation of shoots on the illuminated side, and roots on the 

 shaded portion. That gravity acts in a similar manner may 

 be shown under other conditions. If a Willow twig is rapidly 

 turned, like the diameter of a wheel (Experiment 68), shoots 

 will be formed near the center of revolution, and roots at 

 the peripheral ends. The symmetry of flowers, according to 

 Vochting's researches, is due to the influence of gravity. 



