58 THE COTTON PLANT IN EGYPT CHAP. 



particular locality between, e.g., two varieties, as can be 

 obtained from plots of any dimensions whatever, while 

 by taking more pairs we increase the precision of our 

 comparison to such an extent that mean divergences of 

 only 5 per cent, cannot be due to accident. 



Having obtained from these and from similar small-saale 

 records some definite expression of the precision of the 

 " observation-row method," and knowing approximately 

 the limitations of plot trials, we are in a position to 

 arrange future tests of varieties, manures, waterings, 

 sowing-distances, sowing-dates, &c., with a definite fore- 

 knowledge of the significance of our results. 



The growth curve. Records of the growth of the 

 main stem have already been discussed with respect to the 

 controlling factors up to the end of June. After this date 

 we begin to perceive remarkable differences between various 

 strains of cotton plants. Thus, on the same piece of land, 

 and in the same season, by weekly or fortnightly measure- 

 ments of the height, made to the nearest five centimetres 

 with a graduated rod, we obtain mean growth curves like 

 those of Fig. 43, plotted from families of King American 

 Upland, Egyptian Afifi, Egyptian Sultani, and com- 

 mercial Afifi. The first ceases to grow during June, the 

 second during August, while the third continues at a fairly 

 uniform growth-rate until October. In the third case we find 

 on examination at the end of the season that the length of 

 the internodes is uniform throughout the adult stem. 

 In the case of American Upland only a few internodes of 

 uniform length are produced, after which some half-dozen 

 of decreasing length result in total cessation of growth 

 from the terminal bud of the main stem. The Afifi strain 

 behaves in the same way, but less suddenly, and at a later 

 date. These compressed internodes contain abnormal 

 quantities of calcium oxalate crystals. 



Until more detailed investigation of the growth-rates of 

 various portions of the same plant has been completed, 



