| → the resulting degrees of 1866 |
| their 1869 |
|
| → and we are 1866 |
| We are thus 1869 |
|
| → as well as 1866 |
| and 1869 |
|
| ↑ 1 blocks not present in 1859 1860 1861 1866 1869; present in 1872 |
| We will now look a little closer at the probable nature of the differences between species which induce sterility in first crosses and in hybrids.
|
|
| → between pure species, 1866 1869 |
| OMIT 1872 |
|
|
when crossed; and bearing in mind that in reciprocal crosses between the same two species there is sometimes the widest difference in
→the resulting degrees of
sterility, we must give up the belief that natural selection has come into
→and we are
driven to our former proposition,
the sterility of first crosses, and indirectly of hybrids, is simply incidental on unknown differences in the reproductive systems of the parent-species. |
|
| We may now try and look a little closer at the probable nature of these differences, which induce sterility in first
→as well as
in hybrids. Pure species and hybrids differ, as already remarked, in the state of their reproductive organs; but from what will presently follow on reciprocally dimorphic and trimorphic plants, it would appear as if some unknown bond or law existed, which causes the young from a union not fully fertile to be themselves more or less infertile. ↑
|
| In the case of first
→between pure species,
the greater or less difficulty in effecting an union and in obtaining offspring apparently depends on several distinct causes. There must sometimes be a physical impossibility in the male element reaching the ovule, as would be the case with a plant having a pistil too long for the pollen-tubes to reach the ovarium. It has also been observed that when
of one species is placed on the stigma of a distantly allied species, though the pollen-tubes protrude, they do not penetrate the stigmatic surface. Again, the male element may reach the female
but be incapable of causing an embryo to be developed, as seems to have been the case with some of
experiments on Fuci. No
can be given of these facts, any more than why certain trees cannot be grafted on others. Lastly, an embryo may be developed, and then perish at an early
|
