320 



STUDIES IN LIFE AND SENSE 



The compass flower, that the finger of God has suspended 

 Here on its fragile stalk to direct the traveller's journey 

 Over the sea-like, pathless, limitless waste of the desert. 



The plant is alleged to set the edges of its leaves directly north and 

 south, and Sir Joseph Hooker adds that, from his observations, 



he believes that the leaves pre- 

 sent their faces parallel to the 

 meridian line. 



The effect of varying light- 

 rays on plant-life presents 

 several interesting features for 

 remark. The varied light-rays 

 of which daylight is composed 

 do not, as might be expected, 

 possess the same effects on 

 plant life and growth. Plant- 

 habit, in a word, again shows 

 itself very markedly in its vary- 

 ing susceptibility to different 

 light-rays. Thus, a green plant 

 largely subsists on the carbonic 

 acid gas which it decomposes, in 

 the presence of light, into its 

 carbon and oxygen, retaining 

 the former for food, and setting 

 free the latter. Now, it has 

 been experimentally proved that, 

 in respect of the influence of the 

 light-rays on this chemical pro- 

 cess, the red and orange rays are 

 most powerful ; next succeed the 

 yellow rays ; the green rays come 

 next in order ; whilst the blue 

 and violet rays rank as the least 

 powerful in the scale. But if the yellow rays are the most powerful 

 in aiding the plant to obtain its carbon-food from the air, these rays 

 are least effective in producing mechanical alterations in plant- 

 structure. For it is the refrangible violet rays which in the formation 

 of plant-habit have operated most powerfully in the production of 

 plant-movements, whilst the red rays have no effect. When stems 

 and branches are influenced by and drawn towards the light, the 

 blue and violet light- rays are paramount. On sensitive plants, these 

 rays also exert a stimulating action, but the red and orange rays 

 cause such plants to assume the position and attitude customary to 

 them in darkness. 



FIG. 30. LEAVES OF SENSITIVE PLANT 

 (expanded above and closed below). 



