Chap. XII. CONCLUDING EEMAEKS. 567 



mitted beneath the soil to a depth where no light 

 enters. It follows from this localisation, that the 

 lower parts of the cotyledons of Phalaris, &c., which 

 normally become more bent towards a lateral light 

 than the upper parts, may be brightly illuminated 

 during many hours, and will not bend in the least, if 

 all light be excluded from the tip. It is an interest- 

 ing experiment to place caps over the tips of the 

 cotyledons of Phalaris, and to allow a very little light 

 to enter through minute orifices on one side of the 

 caps, for the lower part of the cotyledons will then 

 bend to this side, and not to the side which has been 

 brightly illuminated during the whole time. In the 

 case of the radicles of Sinapis alba, sensitiveness to 

 light also resides in the tip, which, when laterally 

 illuminated, causes the adjoining part of the root to 

 bend apheliotropically. 



Gravitation excites plants to bend away from the 

 centre of the earth, or towards it, or to place them- 

 selves in a transverse position with respect to it. 

 Although it is impossible to modify in any direct 

 manner the attraction of gravity, yet its influence 

 could be moderated indirectly, in the several waj"s 

 described in the tenth chapter; and under such 

 circumstances the same kind of evidence as that given 

 in the chapter on Heliotropism, showed in the plainest 

 manner that apogee tropic and geotropic, and probably 

 diageotropic movements, are all modified forms of 

 eircumnutation. 



Different parts of the same plant and different 

 species are affected by gravitation in widely different 

 degrees and manners. Some plants and organs exhibit 

 hardly a trace of its action. Young seedlings which, 

 as we know, circumnutate rapidly, are eminently sensi- 

 tive ; and we have seen the hypocotyl of Beta bending 

 25 



