230 GENETICS IN RELATION TO AGRICULTURE 



these cases with some reservations, looking to the future for experimental 

 investigations which will provide us with a satisfactory explanation for 

 them. 



The Vigor of Species Hybrids. The increased vigor displayed by 

 species hybrids has been frequently commented upon by investigators 

 from the time of Kolreuter down to the present. In 1849 Gartner in his 

 general treatment of this subject in species crosses especially notes that 

 the luxuriance of hybrids frequently expresses itself in an unusual develop- 

 ment of practically all plant parts. He also cites a considerable num- 

 ber of the earlier hybridists who have noted this increased vigor, among 

 them Kolreuter, Sageret, Berthollet, Herbert, Mauz, and Lecoq. Hy- 

 brids which up to that time had been particularly noted for this sort of 

 vigor represented such a large number of different families that there could 

 be no question as to the generality of the phenomenon. For increase in 

 length of stem Gartner notes especially Verbascum lychnites X V. thapsus 

 which grows to a height as great as 15 feet; Althcea cannabina X A. 

 officinalis which sometimes attains a height of 12 feet; Malva mauritiana 

 X M. sylvestris which attains a height of 11 feet; Digitalis purpurea X D. 

 ochroleuca which grows to a height of 10 feet; and finally Petunia nyctagini- 

 flora X P. phoenicea and Lobelia cardinalis X L. syphylitica which attain 

 a height of 3 to 4 feet, a significant increase as compared with their 

 low-growing parents. Often the vigor is expressed in a general increase 

 in size throughout as appears to be particularly true of hybrids between 

 different species in the genera Mirabilis and Datura. In Nicotiana a 

 number of hybrids such as N. suaveolens X N. macrophylla, N. rustica X 

 N. marylandica, and many others display such general hybrid vigor 

 sometimes to a very marked extent. Tropceolum majus X T. minus, a, 

 hybrid of the tall and dwarf nasturtiums of the garden is another notable 

 instance of hybrid development. Gartner also records many interesting 

 ways in which this hybrid vigor expresses itself. Thus certain hybrids 

 in Dianthus, Lavatera, Lobelia, Lychnis, Geum, and Penstemon while 

 not displaying notable increases in vegetative vigor lend themselves 

 much more readily to vegetative propagation th&n do their parents. In 

 some cases the hybrids show an unusual tendency to produce side 

 branches and suckers, and in other cases still other outlets of this hybrid 

 vigor are found. 



Not all species hybrids, however, display hybrid vigor, and many 

 indeed show a strikingly weakened condition accompanied by much less- 

 ened vegetative vigor. In tobacco several species hybrids show less- 

 ened vegetative vigor, as for example, Nicotiana grandiflora X N. gluti- 

 nosa, N. glutinosa X N. quadrivalvis, N. rustica X N. suaveolens, and N. 

 suaveolens X N. quadrivalvis. Similarly Verbascum blattaria X 'V. lych- 

 nitis gives weakened hybrids. Consequently within the same genus 



