CHAP. XII. CONCLUDING EEMAEKS. 5C9 



reformed ; and this was then acted on by geotropism, 

 and the radicle became curved perpendicularly down^ 

 wards. 



It has now been shown that the following important 

 classes of movement all arise from modified circum- 

 nutation, which is omnipresent whilst growth lasts, 

 and after growth has ceased, whenever pulvini are 

 present. These classes of movement consist of those 

 due to epinasty and hyponasty, those proper to 

 climbing plants, commonly called revolving nutation, 

 the nyctitropic or sleep movements of leaves and 

 cotyledons; and the two immense classes of move- 

 ment excited by light and gravitation. When we 

 speak of modified circumntitation we mean that light, 

 or the alternations of light and darkness, gravitation, 

 slight pressure or other irritants, and certain innate 

 or constitutional states of the plant, do not directly 

 cause the movement ; they merely lead to a tempo- 

 rary increase or diminution of those spontaneous 

 changes in the turgescence of the cells which are 

 already in progress. In what manner, light, gravita- 

 tion, &c., act on the cells is not known ; and we 

 will here only remark that, if any stimulus affected 

 the cells in such a manner as to cause some slight 

 tendency in the affected part to bend in a beneficial 

 manner, this tendency might easily be increased 

 through the preservation of the more sensitive indi- 

 viduals, But if such bending were injurious, thft 

 tendency would be eliminated unless it was over- 

 poweringly strong; for we know how commonly all 

 characters in all organisms vary. Nor can we see any 

 reason to doubt, that after the complete elimination of 

 a tendency to bend in some one direction under a 

 certain stimulus, the power to bend in a directly 



