1912] STEVENS—HETEROSTYLOUS PLANTS 281 
shows slight differences in shape due to the different position of the 
anthers. In Pulmonaria there is also a slight difference in the size 
of the corolla, and in Pontederia in its color. The most striking 
differences, of course, occur in the stamens and pistils (p. 247). 
The pistils differ in length of style and in size and shape of stigma. 
The stamens show a corresponding difference in length of filament 
or in place of insertion; and there is sometimes a difference in the 
color and thickness of the filaments, and in the size of the anthers. 
The pollen grains usually show a marked difference in size in the 
different forms. Of 43 cases cited, only 8 showed pollen grains 
of equal size in the different forms. In all the others the size of 
the pollen in the short-styled form exceeded that of the long-styled 
form (p. 249). The greatest difference was found in a tri- 
morphous species of Pontederia, in which the diameters of the pollen 
grains from the longest stamens are to those of the shortest as 
100:55, indicating a difference in contents in the ratio of 6:1. 
These THOR ROME E differences pak slight, however, compared 
with the physiological differences whi ythem. Repeated 
experiments have shown that cemclets fertility in heterostylous 
plants is secured only when a flower is pollinated with pollen from a 
flower of another form, that is, the pollen must come from a stamen 
equal in length to the pistil on which it is placed. The superiority 
of legitimate over illegitimate pollination is shown by the propor- 
tion of flowers which yield capsules and by the average number 
of seeds per capsule (p. 245). As DARWIN himself states, mor- 
phological characters alone do not furnish conclusive evidence of 
heterostyly. Final proof can be derived only from experiments 
which show that pollen must be applied from one form to the other 
in order to insure complete fertility. 
The physiological difference in the forms is exhibited also in the 
time necessary to secure fertilization with legitimate and illegiti- 
mate pollen. Darwin (p. 31) placed on several stigmas of a long- 
styled cowslip plenty of pollen from the same plant, and after 24 
hours added some from a short-styled dark red polyanthus. From 
the flowers thus treated, 30 seedlings were raised, and all, without 
exception, bore reddish flowers. Darwin describes a still more 
striking difference in Linum perenne (p. 87). He placed pollen 
