150 INBREEDING AND OUTBREEDING 



The manifestations of heterosis are most noticeable 

 as increases in size. This gain in size in plants which are 

 more or less determinate in their number of parts is made 

 up of an increase in the size of parts rather than in the 

 number of parts. In maize the number of nodes is in- 

 creased much less in comparison to length of internodes. 

 For example, in a large series of crosses between inbred 

 strains of maize height of plant on the average advanced 

 27 per cent., whereas the number of nodes rose only 6 

 per cent. Corresponding to the increase in internode 

 length there is an extension in diameter of stalk, length 

 and breadth of leaves. Root development is proportion- 

 ally augmented. Both the tassels and ears are larger, and 

 frequently two ears develop on crossed plants where 

 either parent produces one, the color of the foliage tes- 

 tifying to the greater vigor. 



The greatly enhanced growth of a plant may be made 

 up by increase in the size of cells, as well as by a multi- 

 plication in the number of cells. However, in a cross 

 between different species of Catalpa no differences could 

 be seen in tracheid length, although the cross was con- 

 siderably taller and larger in diameter. 



The principal effect of crossing maize is shown by an 

 additional production of seed. A number of crosses have 

 given 180 per cent, increases in yield of grain over their 

 inbred parents. Examples of what can be done are seen in 

 the accompanying illustrations (Figs. 31 and 32). Im- 

 provement in yield is shown by crosses between inbred 

 strains derived originally from the same variety, as well 

 as between crosses of strains derived from different vari- 

 eties or even from quite distinct types. The results have 

 been very wonderful as a whole, giving at the very least 



