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 Kélreuter, 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; Althea cannabina X A. 
officinalis which sometimes attains a height of 12 feet; Malva mauritiana 
< M. sylvestris which attains a height of 11 feet; Digitalis purpurea X D. 
ochroleuca which grows to a height of 10 feet; and finally Petunza nyctagtni- 
flora X P. phenicea 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 & N. macrophylla, N. rustica X 
N. marylandica, and many others display such general hybrid vigor 
sometimes to a very marked extent. Tropeolum majus X T. minus, a 
hybrid of the tall and dwarf nasturtiums of the garden is another notable 
instance of hybrid development. Gjirtner 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 than 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 «'V. lych- 
nitis gives weakened hybrids. Consequently within the same genus 
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