INTKODUCTION. 5 



attraction of gravity being transmitted from the ex- 

 cited part for some little distance along the organ in 

 question, have an important bearing on the theory of 

 all such movements. 



Terminology. — A brief explanation of some terms which will 

 be used, must here be given. With seedlings, the stem which 

 supports the cotyledons (i.e. the organs which represent the first 

 leaves) has been called by many botanists the hypocotyledonous 

 stem, but for brevity sake we will speak of it merely as the 

 hyyocotyl : the stem immediately above the cotyledons will be 

 called the epicolyl or plv/mule. The radicle can be distinguished 

 from the hypocotyl only by the presence of root-hairs and the 

 nature of its covering. The meaning of the word circumnu- 

 tation has already been explained. Authors speak of positive 

 and negative heliotropism,*— that is, the bending of an organ 

 to or from the light ; but it is much more convenient to confine 

 the word heliutrapism to bending towards the light, and to 

 designate as ajheliutropism bending from the light. There is 

 another reason for this change, for writers, as we have 

 observed, occasionally drop the adjectives positive and negative, 

 and thus introduce confusion into their discussions. Didhelio- 

 tropism may express a position more or less transverse to 

 the light and induced by it. In like manner positive geotrff- 

 pism, or bending towards the centre of the earth, will be 

 called by us geotropism ; apogeotropixm will mean bending in 

 opposition to gravity or from the centre of the earth ; and dia- 

 jeotropism, a position more or less transverse to the radius of 

 the earth. The words heliotropism and geotropism properly 

 mean the act of moving in relation to the light or the earth ; 

 but in the same manner as gravitation, though defined as " the 

 act of tending to the centre," is often used to express the cause 

 of a body falling, so it will be found convenient occasionally to 

 employ heliotropism and geotropism, &c., as the cause of the 

 movements in question. 



The term epinasty is now often used in Germany, and implies 

 that the upper surface of an organ grows more quickly than the 



• The highly useful terms of Frank : see his remarkable ' Bei- 

 Heliotropism and Geotropism tiage zur Pflanzenphysiologio, 

 were first used by Dr. A. B. 1868. 



