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 became sterile; now it is fertile[400] if kept in a dry stove during the winter. Other varieties of pelargonium are sterile and others fertile without our being able to assign any cause. Very slight changes in the position of a plant, whether planted on a bank or at its base, sometimes make all the difference in its producing seed. Temperature apparently has a much more powerful influence on the fertility of plants than on that of animals. Nevertheless it is wonderful what changes some few plants will withstand with undiminished fertility: thus the _Zephyranthes candida_, a native of the moderately warm banks of the Plata, sows itself in the hot dry country near Lima, and in Yorkshire resists the severest frosts, and I have seen seeds gathered from pods which had been covered with snow during three weeks.[401] _Berberis Wallichii_, from the hot Khasia range in India, is uninjured by our sharpest frosts, and ripens its fruit under our cool summers. Nevertheless I presume we must attribute to change of climate the sterility of many foreign plants; thus the Persian and Chinese lilacs (_Syringa Persica_ and _Chinensis_), though perfectly hardly, never here produce a seed; the common lilac (_S. vulgaris_) seeds with us moderately well, but in parts of Germany the capsules never contain seed.[402]