HELIOTROPES, OR SUNFLOWERS. 179 



horizontally. " If," says Mr. Darwin, " the two (seed- 

 leaves) are placed in the line of entering light, the 

 one farthest from it rises up and that nearest to it 

 often sinks down ; if placed transversely to the light 

 they twist a little laterally, so that in every case they 

 endeavour to place their upper surfaces at right 

 angles to the light. So it notoriously is with the 

 leaves on plants nailed against a Avail or grown in 

 front of a window. A moderate amount of light 

 suffices to induce such movements ; all that is 

 necessary is, that the light should steadily strike 

 the plants in an oblique direction."^ 



It must be borne in mind that in determining the 

 range of sleep in plants, an artificial limit of rise or 

 depression had to be accepted, and this was taken 

 at 60° above or below the horizon. Consequently 

 there would be a number of plants which would not be 

 ranged with " sleepers," simply because the elevation 

 or depression of their leaves or leaflets uniformly fell 

 short of 60°. It is probable that some of these 

 instances would fall appropriately under " transverse 

 heliotropism " if their movements were accurately 

 determined. There can be no doubt that all these 

 motions have an intimate relationship to each other, 

 and it is contended that all are modifications of one 

 motion, namely, that of circumnutation. 



' Darwin, " Movements of Plants," p. 439. 



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