It has also been observed that when the pollen 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 element, but be incapable of causing an embryo to be developed, as seems to have been the case with some of Thuret's experiments on Fuci.

No explanation 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 period.

This latter alternative has not been sufficiently attended to; but I believe, from observations communicated to me by Mr.