210 MENDELISM 
(long pollen), in the other it is approximately spherical 
(round pollen). Sweet-peas normally undergo self- 
pollination, so that the two types naturally remain 
distinct. Let us see what happened when the long- 
and the round-pollined forms were crossed together. 
The cross-bred plants (F,) had coloured flowers — 
flowers of the old-fashioned purple type known to 
florists as Purple Invincible, which is characterized by 
a purple standard and blue wings. The pollen pro- 
duced by these plants was of the long type. Thus as 
regards the shape of the pollen grains there was simple 
dominance. But the union of two white-flowered 
types has given rise to a series of plants all possessing 
a definite colour character—purple with blue wings. 
This character is very probably the same as that 
exhibited by the common ancestor of all our cultivated 
sweet-peas. Here, then, we seem to have a clear case 
of reversion to the ancestral type on crossing. We 
shall find that the Mendelian principles will enable us 
to arrive at a clear conception of the mechanism of 
this process. 
The cross-bred plants were self-pollinated, and in F, 
the following types made their appearance in approxi- 
mately the proportions given ; 
Purple Invincible ... 81 3 
or 
Picotee _ wee a 
Painted Lady ee =. 277) 3 
} or 
Tinged white ee 9 
White... see we 112 7 
