108 THE AMERICAN BOTANIST 



the upper parts of the root, causing a henchng away from the 

 affected side. The tip also can distinguish between slightly 

 harder and softer objects to a marked sensibility. One might 

 compare the tip of the radicle to the nervous system of an- 

 mials. — it receives the impressions and then by some marvelous 

 methods directs the organism. 



Several other factors which will be discussed later arc 

 active in determining the direction of growth. In almost everv 

 case however, the final purpose or advantage of several move- 

 ments is obviously for the life of the plant. 



After the radicle has penetrated the soil, the hvi)(>ci)tvls of 

 the dicotyledonous seeds break up thru the ground in the form 

 of an arch. When the cotyledons are hypogean, that is, re- 

 main buried in the soil, the hypocotyl is hardly developed and 

 the epicotyl or plumule raises in an arched manner. 



The cotyledons after they have broken thru the ground 

 are in constant motion, chiefly in a vertical plane and com- 

 monly once up and down in the course of twenty four hours. 

 How-ever, there are many exceptions to such a simplicity of 

 movement. Their upward and downward movements do not 

 exactly conicide, therefore generating an ellipse. In many 

 cases it is found that the cotyledons sink downward a little in 

 the forenoon and rise in the afternoon or evening. Thus they 

 stand rather more highly inclined during the night than during 

 the mid-day, at which time they are expanded almost hori- 

 zontally. The circumnutating movement is thus at least 

 partially periodic, no doubt in connection with the daily alter- 

 nations of light and darkness. 



The conclusions reached from the study of circumnuta- 

 tion is that it was not gained for any special purpose, because 

 it is so universal a phenomena; but that it follows in some 

 unknown wav from the manner in which vegetable tissues 



