416 BOTANICAL GAZETTE [JUNE 
The older plants, subjects of the preceding experiment, were 
growing so rapidly that the first observation (after five and a 
half hours) was too late to show the gradual retreat from the 
tip of the region of curvature. But in roots it always happens,® 
as in the base of the stem in these very young plants, that the 
curve first appears near the tip and gradually moves farther away. 
Of course if the growth is rapid so will be the development of 
the curve. 
In the base of the hypocotyl, then, the downward curve 
occurs only in a zone not separated from the root tip by any 
mature tissue, and not at most as remote from the tip as the 
extreme limit of what may fairly be considered the apical grow- 
ing region. In these points, and in all other visible details of 
the execution of the response, it agrees with the familiar mani- 
festations of positive geotropism in roots. From this and the 
decapitation experiments, and from the subsequent elimination 
of the curve by later growth, the conclusion 
| seems obvious that the response is to a 
stimulus received at the root tip. 
Similar but less complete observations 
have been made on Robinia Pseudacacia, 
Helianthus annuus, Cucurbita Pepo, Eucglyptus 
globulus, and Pinus ‘Sabian (fig. 2): The 
embryos of Robinia, with radicles 0.5™™, and 
hypocotyls 2.0™ in length, were fastened 
horizontal. In one day the hypocotyl 
became 5.4™™" (average of ten plants) long, 
and the root 5™™, the curve of least radius 
Fic. 2.—Young seedling being 3™" above the line of separation, that 
of Pinus Sabiniana, show- is, it was above the middle of the hypocotyl. 
ing downward curve in Two plants were placed in water culture and 
wiih the curve grew out. My observations 0? 
little to form the arch the anatomy have no value here, for the 
ses the cotyle- reason explained in the treatment of Lupi- 
eae nus. Decapitation was tried on Helianthus 

®Sacus, /. c., pp. 831, seg. 
. 
a 

