32 THE COTTQN PLANT IN EGYPT CHAP. 



the open air. Daily measurements of height were made, 

 and the final length of the hypocotyl when growth in this 

 region had ceased showed that the Upland exceeded the 

 Egyptian by 50 per cent, under the field conditions of an 

 Egyptian April. This excess was the more striking, in that 

 the Upland seed was much smaller than the Egyptian, and 

 a large seed usually displays a stronger " field germina- 

 tion" than a small one. 23 



This difference became still more remarkable when the 

 first internode developed above the cotyledons had also 

 attained to its full length, for it completely reversed the 

 hypocotyl length, being 50 per cent, longer in the Egyptian 

 than in the Upland. The length of subsequent internodes 

 is correlated with that of the first one, and since the 

 internodes are developed at about the same time, the 

 height of the plant is determined by the internode length 

 unless other factors intervene, which we shall consider later. 



A repetition of this experiment in a cool greenhouse in 

 England threw some light on the specific temperature- 

 relationships. Under these conditions the Egyptian 

 seedlings exceeded the American in all their dimensions, 

 including the hypocotyl length. 



More detailed examination of the water-relationships 

 which we have already encountered in the " sunshine 

 effect " are now necessary, dividing the same into the two 

 components, namely, absorption by the root and loss by 

 the stem. 



The root. The root first makes its appearance in 

 germination at the end of the emerging hypocotyl as a 

 zone of embryonic tissue about three millimetres in length. 

 The growing zone is remarkably small throughout root- 

 development. 



The primary root is positively geotropic, and its elonga- 

 tion is indefinite. The plant is consequently tap-rooted, 

 and the depth to which the root system extends is deter- 

 mined by external conditions alone. The greatest depth 



