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, eg. 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.5 
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 
