Correlation 97 



in the seed. More commonly, however, in older plants and in cuttings, a 

 rather close balance becomes established between root and shoot and is 

 restored if altered experimentally. 



This ratio is subject to change during development, for in most plants 

 the shoot grows consistently faster than the root. Other factors also affect 

 it. In poorly nourished plants the root is relatively large and in etiolated 

 ones, relatively small. Crist and Stout (1929) found that in some plants 

 it was affected by soil acidity, soil fertility, and day-length. Roberts and 

 Struckmeyer (1946) observed that temperature and photoperiod modified 

 the ratio but not in the same way in all plants. The top-root ratio was 

 studied by Shank (1945) in maize inbreds with low and with high ratios, 

 and in their hybrids, under different amounts of phosphorus, nitrogen, and 

 water in the soil. Increase in each of these substances tended to increase 

 this ratio. Richardson ( 1953 ) measured root growth microscopically in 

 small maple seedlings growing in glass tubes under controlled conditions. 

 Any change in the environment of the shoot which modified photosyn- 

 thetic activity had a commensurate effect on rate of root growth. Correla- 

 tions depending on nutrition are evidently rather susceptible to change 

 by environmental factors. 



The influence of shoot on root is not always nutritional but may result 

 from the action of auxin, vitamins, or other growth-regulating substances. 

 The nutritional influence of root on shoot is well shown by the horticul- 

 tural practice of producing dwarf trees by grafting scions from normal- 

 sized varieties on roots of genetically dwarf types in which the root system 

 is too small to supply the growth requirements of a large tree. 



Among other correlations which have their basis in nutritional factors 

 are those between the size of a fruit and the amount of leaf area avail- 

 able for the support of its growth (Haller and Magness, 1925). There is 

 also a close relation between the amount of foliage on a tree (the size 

 of its crown ) and the amount of stem growth. Young and Kramer ( 1952 ) 

 and Labyak and Schumacher (1954) have studied this problem by re- 

 ducing experimentally the size of the crown in pine through pruning and 

 observing the effect on trunk growth. In apples, Murneek (1954) found a 

 relationship between fruit size and leaf area (presumably nutritive) and 

 also between fruit size and seed number per fruit (presumably stimu- 

 latory ) . 



Because of its practical importance, many studies have been made of 

 the relation between the size of seed planted and the size of the plant 

 growing from it. If a positive correlation existed between these characters 

 it would pay to use only large seeds in many agricultural operations. 

 Agronomists have sought all such characters in fruit and seed that might 

 be correlated with high yield but have had little success. Where such a 

 relation has been found, in most cases it is simply between seed size and 



