074 
POLLINATION OF FLOWERS 
stock shows little improvement over self- 
fertilization. The cross must be between 
two different races or strains, or between 
two species. 
When two weak, run-out, self-fertilized 
strains of corn, which yielded at the rate 
of only 29 bushels to the acre, were crossed 
the hybrids yielded at the rate of 68 bush¬ 
els to the acre. Thus, in cultivation of many 
crops, the crossing of two weak varieties 
may be very profitable. Hybrids between 
two different species of oaks and two spe¬ 
cies of elms were in eight years one-third 
larger than the parent species. 
The evil effects of inbreeding have long 
been known in a general way, and are even 
instinctively recognized by barbarous races; 
but they are not so easily recognized as the 
good effects, for the deterioration is grad¬ 
ual. When corn is self-fertilized for a 
number of generations the plants become 
inferior in size, vigor, and productiveness, 
as compared with normally crossed plants 
from the same source. Many plants have 
become self-sterile to their own pollen, 
which would seem to be evidence in favor 
of cross-pollination. 
A summary of the more important ways 
in which self-pollination, and consequently 
self-fertilization, is prevented in plants is 
given in the following table: 
A. SEPARATION OF STAMENS AND PISTILS BY 
SPACE. 
1. Stamens and pistils in different flow¬ 
ers, but on the same plant. 
2. Stamens and pistils in different flow¬ 
ers and on different plants. 
3. Stamens longer than the pistils, or 
pistils longer than the stamens. 
4. Stamens bent away from the pistils, 
or pistils bent away from the stamens. 
B. SEPARATION OF STAMENS AND PISTILS BY 
- TIME. 
1. Anthers mature before the stigmas. 
2. Stigmas mature before the anthers. 
C. MECHANICAL SEPARATION OF THE STA¬ 
MENS AND PISTILS. 
1. Absolute separation. Anthers or pol¬ 
len masses held in a fixed position, and 
never set free unless the flower is visited 
by insects. 
2. Partial separation. Self-pollination 
may occur during the latter part of the 
blooming period. 
D. PHYSIOLOGICAL SEPARATION OF THE STA¬ 
MENS AND PISTILS. 
1. Stamens aborted in some flowers, pis¬ 
tils in others. 
2. Pollen from a different flower prepo¬ 
tent over pollen from the same flower. 
3. Pollen from the same flower sterile or 
impotent on its own stigma. 
“Crossing is by no means confined to the 
races and varieties of the same species, but 
is very common between different species 
and may occur between different genera. It 
is rapidly coming to be regarded as an im¬ 
portant factor in evolution, and Lotsy re¬ 
gards it as the most important agency in 
the development of the plant world. In 
Kerner’s time more than 1,000 hybrids 
were known in the flora of Europe, and 
he fully believed that many new species 
originated in this way. Darwin had pre¬ 
viously realized the possibility that hy¬ 
bridism might have played an important 
part in the history of evolution; but ow¬ 
ing to the general belief that hybrids 
were almost invariably sterile he underes¬ 
timated its significance, altho he observed 
that every intermediate stage existed be¬ 
tween complete sterility and complete fer¬ 
tility.” 
“While hybrids do in general show de¬ 
creased fertility, there are thousands of 
cases in which they multiply readily by 
seed. Jeffrey has recently shown that hy¬ 
brids among the Angiosperms, or flowering 
plants, are characterized by having a part 
of the pollen imperfect or aborted; and, 
judged bjr this test, they are very common 
both among wind-pollinated and insect- 
pollinated plants. A great many forms 
which have long been regarded by systema- 
tists as perfectly good species are now rec¬ 
ognized by their aborted pollen as hidden 
hybrids. They are especially abundant 
among the roses, apples, pears, brambles, 
hawthorns, mulleins, gentians, nightshades, 
evening-primroses, thistles, hawkweeds, and 
asters. Among wind-pollinated flowers hy¬ 
brids are very common in the sedges, 
rushes, pondweeds, oaks, and birches.” 
(Lovell, J. H. The Flower and the Ree; 
Plant Life and Pollination, p. 277.) 
