WORK OF THE SHOOT 77 



or retarded growth ? These experiments show that shoots 

 are sensitive to light, and that their growth is retarded 

 by it. Thus shoots turn to the light because they grow 

 more quickly on the shaded than on the illuminated side. 

 Such movements of plants due to the stimulus of light are 

 called heliotropic movements. And since shoots turn 

 towards the light they are positively heliotropic. Note 

 that the leaves also turn their blades towards the light. 

 The leaves of some plants, however, grow erect and expose 

 their tips and edges to the light, e.g. the Iris, Daffodil, and 

 Hyacinth ; this habit is common in plants growing in very 

 sunny situations. 



Rates of growth in light and in darkness. Take two pots 

 of seedling Beans, allow one to grow for two or three weeks 

 under ordinary conditions of light, and the other for a similar 

 time in the dark (Fig. 47, 1 add 2). Compare them as to 

 colour, length of internodes, and size of leaf. Measure them 

 day by day, compare their behaviour and plot the results 

 in curves on squared paper as in Fig. 48. 



Etiolation. Conditions necessary for the formation of 

 chlorophyll. From these observations we learn that shoots 

 which develop in the dark are yellowish-white in colour, 

 greatly elongated and tender, and that the leaves are much 

 smaller than normal. These changes brought about by 

 growth in darkness are known as etiolation (Fr. e'tioler = to 

 blanch). Shoots grown under ordinary conditions of light 

 develop the green pigment chlorophyll, and have tougher 

 tissues and larger leaves. Normally, chlorophyll is not 

 developed in plants from which light is excluded, but cases 

 of the contrary are not rare, e. g. Pine seedlings develop 

 chlorophyll in the dark, and the embryo of the Sycamore 

 is green while still enclosed within the thick and opaque 

 fruit-coat. 



Not only is light usually necessary for the formation of 

 chlorophyll, but we learned from water-culture experiments 



