GROWTH. 73 



Light also influences the form and size of the cells, as well as 

 of the entire plant. (See § 40.) 



EXPERIMENT 92. 



GROWTH OF SEEDLINGS IN DARKNESS. 



Grow seedlings of Cucurbita (Squash) in similar pots, some of 

 which are set in the light, and others pj(;_ g. 



are covered by a pasteboard box, at 

 the same temperature. The latter do 

 not develop normally ; the shoot axes 

 are much extended, and form only im- 

 perfect leaves, which assume an up- 

 right position. (Fig. 67.) All parts 

 of the plant are pale and dispropor- 

 tionately tender. The lignification of 



the wood is hindered, in consequence ^^eurbitaseeQungs. (Detmer.) 

 of which there is no opposition to the «, grown in darkness ; b, 

 extension of the tissues by the turgor grown in light, 

 stretching of the parenchyma-cells. 



Bemark. — Care must be taken in this experiment that both plants do 

 not stand in the sunlight, otherwise an abnormally high temperature will 

 arise in the pasteboard box, and thus the relations of temperature will be 

 altered. 



EXPERIMENT 93. 



COMPARISON OF GROWTH OF SEEDLINGS IN LIGHT AND DARKNESS. 



Germinate a number of Peas in a pan of moist sawdust until the 

 main roots are 2 cm. long. After the roots of several, as nearly 

 alike as possible, have been marked with a scale, as in Experiment 

 89, place some in the light, and others under a pasteboard box, over 

 spring-water. It will be found that the growth in light is less than 

 in darkness. At the same time the daily period of growth can be 

 observed. 



55. Influence of Light upon the Anatomy of the Leaf — The 

 leaves of common trees have in the upper side a closely- 

 arranged layer of cells rich in chlorophyll {palisade parenchyma), 

 and in the lower side a loosely-arranged tissue poor in chloro- 

 phyll {spongy parenchyma). This arrangement depends upon 

 the influence of light. Shaded leaves exhibit another struc- 



