174 FREAKS OF PLANT LIFE. 



of precision, distinct terms have been applied. In the 

 first place, there are plants which when exposed to a 

 strong lateral light turn speedily towards it. This is 

 true heliotropism, or turning towards the surr. Then, 

 secondly, there are plants which, when exposed in a 

 similar manner to a bright side-light, manifestly and 

 speedily turn from it. This has been called " negative 

 heliotropism," but the term which Mr. Darwin has 

 employed to designate such movements is aphelio- 

 tropism. Thirdly, there are plants which on exposure 

 to light, when sufficiently intense, place their leaves 

 transversely to the direction whence the light proceeds, 

 and this has been called "transverse heliotropism,"' 

 or, as Mr. Darwin prefers to call it, diaheliotropism. 

 And finally, there are those plants which direct their 

 leaves by rising, or falling, or twisting, so that they 

 may be less intensely illuminated, and these move- 

 ments, which are sometimes called " diurnal sleep," 

 the same author designates as paraheliotropism. It 

 is clear that if grouped in accordance with their 

 purposes the first and third of these types are allied, 

 as also are the second and fourth. In " heliotropism " 

 and " transverse heliotropism " the object is to turn 

 into and take advantage of the light. In " negative 

 heliotropism " and " diurnal sleep," on the contrary, 

 the object seems to be turning from, or shunning as 

 much as possible, the direct influence of the light 

 We have given above the four terms by which it is 



