Theory of Plant Breeding 



be quite simply explained as the usual changes which 

 most new introductions go through when grown under 

 strange and new conditions. 



Many of De Vries' characters, e.g. giant and dwarf 

 forms, colour of flowers, mode and time of flowering, 

 are undoubtedly affected by external conditions. But 

 it is, of course, quite well known that sports or sudden 

 changes do appear sometimes without any assignable 

 cause.^ 



This is well marked in many garden plants which are 

 apt to show extra petals, or become double, or to de- 

 velop strangely twisted or flattened stems. 



On well-nourished plants of toadflax, for instance, an 

 extra petal may appear suddenly and the accomplish- 

 ment may be inherited.^ 



Double flowers are perhaps due to a disease set up 

 by parasitic insects or fungi. M. Molliard claims to 

 have produced double flowers in Knautia arvensis and 

 in Matricaria inodora by infecting them with a parasitic 

 fungus (Peronospora violacea and P. radii). He also 

 got Primula officinalis to form double flowers by means 

 of a root-fungus ; other plants were also doubled by 

 infecting them with eel-worms.^ 



This explains the statement, strongly insisted on by 

 Mr. Fenn,* that single flowers are apt to turn double 

 when grown alongside the latter. An insect could very 

 well convey the fungus spores from the double to the 

 single flowers. Extra nourishment is also said to favour 

 the production of doubles. 



When once formed the tendency certainly seems to 

 be inherited, for about 90 per cent, of M. Vilmorin's 

 seedlings inherited the character.^ 



An old recipe given by a Dutch florist about 1499 

 gives a method of making anemone flowers double. 



* See R. H. S. Conference on Genetics, 1906. 

 299 



