148 INBEEEDINa AND OUTBREEDING 



as the invigoration immediately apparent after crossing. 



The effects of outbreeding, unlike those of inbreed- 

 ing, are shown both by plants which are naturally self- 

 fertilized and by those which are cross-fertilized. Many 

 of the illustrations already given are from plants almost 

 invariably self -fertilized. Crossing within a pure line of 

 such a species shows no heterosis; but if the parents 

 united in the cross differ more or less in minor external 

 features an increase in growth is usually to be ex- 

 pected. This has been shown to be true for peas, toma- 

 toes, tobacco and many other normally self-felrtilized 

 forms among cultivated plants, as well as for several 

 wild species. 



An extensive series of crosses between different Nico- 

 tiana species has been reported by East and Hayes.®® 

 The majority of these crosses were taller than the average 

 of the two parents and many were taller and more vigor- 

 ous than either parent. Some of the crossed plants were 

 completely sterile. In certain cases these were weak, non- 

 vigorous plants, but there were others in which inability 

 to produce seed was accompanied by increased vigor. 

 Thus, whUe occasionally the increased development pf 

 sterile hybrids may be due in part to their having ex- 

 pended rio energy in seed production, the fact that many 

 vigorous hybrids manifest greater ability to produce seed 

 shows this is a relatively unimportant factor and entirely 

 inadequate to account for the great vigor obtained where 

 there is full fertility. 



A fair example of the way in which height is gained 

 by crossing is given by East and Hayes,®' a cross of Nico- 

 tiana rustica hrasilia Comes and A^. rustica scdbra Comes. 



