CIRCUMNUTATION IN SEEDLINGS. 403 



1046. Circunmutation iii seedlings. That part of the axis which 

 is below the cotyledons is made up of a rudimentary stem known 

 as the caulicle or hypocotyl, and a rudimentary root or radicle 

 proper. The part of the young stemlet above the cotyledons is 

 termed the epicotyl. In the cotyledons of the plantlet, when 

 freed from the seed-coats, and in all parts of the young axis, 

 slight movements can be observed. In all observations it is 

 necessaiy to remove the plantlet as far as possible from disturb- 

 ing conditions ; thus, all light must be excluded until the moment 

 of making the observation, w r hen only a faint light should be 

 employed. 



1047. Two facts are easily apparent with regard to the revolv- 

 ing radicle: (1) its extreme sensitiveness to contact ; (2) its ten- 

 dency to 3'ielcl to geotropism (see 1026). 



1048. The caulicle, upon emerging from the seed-coats, is 

 often more or less arched ; but it may become straight after a 

 short time, when it can be seen to pass through an elliptical orbit 

 by which the plane of the cotyledons is somewhat inclined suc- 

 cessively to all points of the compass. Darwin has shown that 

 even before the liberation of the caulicle from the seed-coats, when 

 both columns of the arch are held in the soil, the top of the arch 

 moves with considerable regularity. It is difficult to understand 

 how the summit of the arch formed bv the curved caulicle can 



V 



revolve when both of its supporting columns are fixed in the soil. 

 Darwin has accepted an explanation suggested by "\Viesner, 

 which is briefly as follows : In a given internode (it must be 

 remembered that the caulicle represents the first internode of 

 the seedling, as shown in Volume I. page 9) there may be a zone 

 in which the growth is equal on all sides, and which may be 

 termed the zone of indifferent growth, while on each side of this 

 there may be two others in which there is unequal growth at 

 intervals of time. Then by the faster growth on one side of the 

 arch the summit would be thrown to one side, and this process 



is more or less distortion produced by the best methods of projection, and in 

 all accurate observations this must be taken into account. 



When seedlings are inverted so that the glass filament is held upwards, it 

 must be noted that the influence of gravitation must come in as a modifying 

 element. To mark the amount of influence exerted by gravitation, it is well 

 to vary the length and weight of the filament employed. But it must be ob- 

 served that the weight of the organ itself is the most important element in the 

 problem. Moreover, it has been observed that all young growing parts, espe- 

 cially the extremity of the radicle, are more or less sensitive ; and hence the 

 course of the filament may be somewhat modified by even slight contact. 



