106 ME. r. W. KEEBLE ON 



Purtlier, a seedling of Loranthus loniceroides (showing distinct curvature of its dark- 

 grown hypocotyl) exhibits much more curvature when, after preliminary exposure to 

 light, it is allowed to continue its germination in a dark room (woodcut 5 b). Leaving 

 aside the last statement as involving more complicated conditions, I wish to examine 

 this cvirvature. L, loniceroides {cf. fig. 6, PL XI.) has a most convenient hypocotyl 

 for purposes of measurement and observation of change in position, and was therefore 

 selected for experiment. 



In the first place, seeds of Jj. loniceroides w^ere germinated opposite a window and 

 the position of their hypocotyls noted from time to time. As the hypocotyl grew^, it 

 curved so that its head was turned from the light, but later an S-sbaped curvature was 

 set up in the hypocotyl, resulting, in some cases, in tbe pointing of the head of the 

 hypocotyl toward the light {cf. woodcut 6). The hypocotyls of Z. loniceroides, then, do 



-Fig. 6. 



not curve merely in obedience to negative heliotropism ; there is another factor which 

 must be considered, viz. some kind of nutation. That nutation actually occurs was 

 shown by adopting the method devised by Darwin * thus :^The body of a seedling was 

 appropriately fixed, and to the head of its hypocotyl a delicate filament of glass was 

 fastened by means of sealing-wax. This filament bore at either end a minute triangle 

 of paper, the two triangles being so fixed that their surfaces were at right angles to the 

 glass filament. Dots made on a vertical glass plate at points where, on looking through, 

 the two triangles w^ere seen to coincide, indicate the change of position of the head of the 

 hypocotyl. 



The result show^ed that the hypocotyl of X. loniceroides when growing normally is 

 under the directive influence of light ; but also that it is in a condition of nutation. 

 Eurther, this nutation has influence in determining the ultimate curvature of the 

 hypocotyl, and, consequently, the final position to which the head of the hypocotyl is 

 brouo-ht. 



The curvature of the hypocotyl in the dark w^as next investigated. The method 

 described above was adopted. At each registration of the position of the triangles the 

 seedling w^as necessarily exposed, though for very short times, to the light. The 

 objection that the darkness was not absolutely continuous might be urged ; but since in 

 absolutely continuous darkness curvature of the hypocotyl has already been shown to 

 occur, it seems to me fair to assume that the results of this examination are not vitiated 

 by the temporary exposures during the making of the dots on the glass plate. Under 



* ' Power of Movement in Plants,' Introduction, p. 6. 



