oductive individual or flower, and the case may be wholly
different. Nor do I allude to fruit not ripening from want of heat, or
rotting from too much moisture. But many exotic plants, with their
ovules and pollen appearing perfectly sound, will not set any seed. The
sterility in many cases, as I know from my own observation, is simply
due to the absence of the proper insects for carrying the pollen to the
stigma. But after excluding the several cases just specified, there are
many plants in which the reproductive system has been seriously
affected by the altered conditions of life to which they have been
subjected.

It would be tedious to enter on many details. Linnaeus long ago
observed[396] that Alpine plants, although naturally laded with seed,
produce either few or none when cultivated in gardens. But exceptions
often occur: the _Draba sylvestris_, one of our most thoroughly Alpine
plants, multiplies itself by seed in Mr. H. C. Watson's garden, near
London; and Kerner, who has particularly attended to the cultivation of
Alpine plants, found that various kinds, when cultivated, spontaneously
sowed themselves.[397] Many plants which naturally grow in peat-earth
are entirely sterile in our gardens. I have noticed the same fact with
several liliaceous plants, which nevertheless grew vigorously.

Too much manure renders some kinds utterly sterile, as I have myself
observed. The tendency to sterility from this cause runs in families;
thus, according to Gaertner,[398] it is hardly possible to give too
much manure to most Gramineae, Cruciferae, and Leguminosae, whilst
succulent and bulbous-rooted plants are easily affected. Extreme
poverty of soil is less {164} apt to induce sterility; but dwarfed
plants of _Trifolium minus_ and _repens_, growing on a lawn often mown
and never manured, did not produce any seed. The temperature of the
soil, and the season at which plants are watered, often have a marked
effect on their fertility, as was observed by Koelreuter in the case of
Mirabilis.[399] Mr. Scott in the Botanic Gardens of Edinburgh observed
that _Oncidium divaricatum_ would not set seed when grown in a basket
in which it throve, but was capable of fertilisation in a pot where it
was a little damper. _Pelargonium fulgidum_, for many years after its
introduction, seeded freely; it then beca