HELIOTROPES, OR SUNFLOWERS. 173 



Hooker says, that when traversing the prairies with 

 Professor Asa Gray, in 1877, he watched the position 

 of the leaves of many hundred plants from the 

 window of the railway car, and after some time per- 

 suaded himself that the younger, more erect leaves 

 especially, had their faces parallel approximately to 

 the meridian line. At the same time he says that 

 he convinced himself that " the flower-heads of 

 various of the great Helianthoid Compositae, such 

 as that which we call the ' sunflower,' that grew in 

 hosts on the prairies did follow the sun's motion 

 in the heavens to a very appreciable degree, their 

 morning and evening positions being reversed." ^ 



It has been truly said that no one can look at the 

 plants growing on a bank, or on the borders of a 

 thick wood, and doubt that the young stems and 

 leaves place themselves so that the leaves may be 

 well illuminated. Whoever has placed half a dozen 

 plants on a window-sill knows well enough how soon 

 all the leaves and extremities of the branches will be 

 directed towards the window. The tendency of 

 plants to turn to the light is known to every cultivator, 

 but this tendency is more strongly developed in some 

 plants than in others, and in some parts of the same 

 plant than in others. The types of heliotropism, in 

 its broadest sense, are four, and to these, for the sake 



^ J. D. Hooker, in "Gardener's Chronicle," January 15, 1881. 



