121 



If the axis is placed in a vertical position, the stimulus of 

 gravitation is still effective in that it produces an outgrowth which 

 completely surrounds the axis. When the latter is placed more 

 than 6° out of the vertical, no peg is formed upon the upper 

 side. 



The formation of the peg is the result of a peculiar and until 

 recently unrecognized method of geotropic reaction in that the 

 stimulus sets up a growth at right angles to the normal direction 

 of growth, the polarity of which is displaced through an angle 

 of 90 . This change is accompanied in the peg-forming region 

 by a substitution of periclinal for anticlinal divisions. 



2. On the other hand the formation of the peg is conditioned 

 by the bending of the axis, in such a manner that it occurs on 

 the concave side of the curve. In other words, as in the analogous 

 case of the formation of secondary roots on the convex side of 

 the curve of a primary root, the stimulus is derived from the 

 organism itself, and is connected with its form. By appropriate 

 experiment it is possible to separate the two stimuli, and thus to 

 cause two outgrowths to appear on opposite sides. 



Pressure and friction of the axis on the testa do not act as' 

 stimuli. 



Under natural conditions the two stimuli, thus recognizable ex- 

 perimentally, work together in complete harmony with the result 

 that the seedling is successfully freed from the testa at the right 

 time. This process is most successful when the broad faces of 

 seed are placed at right angles to the vertical ; and least success- 

 ful when the micropylar end is directed downwards. 



The final throwing off of the testa, which prevents independent 

 nutrition, occurs usually after 10-14 days without the help of 

 the peg. Climatic conditions naturally affect the results favor- 

 ably or unfavorably. Continuous high temperature of the sub- 

 stratum prevents the effective working of the apparatus by induc- 

 ing a too sudden and rapid lengthening of the hypocotyl. 



In planting, therefore, the seeds of cucurbits should be placed 

 with a broad face directed downward, and the temperature of the 

 substratum should not be kept too high. 



It may be added that the nature of the stimuli, which, as indi- 



