oric
condition. On the other hand, extremely poor soil sometimes, though
rarely, appears to cause doubleness: I formerly described[419] some
completely double, bud-like, flowers produced in large numbers by
stunted wild plants of _Gentiana amarella_ growing on a poor chalky
bank. I have also noticed a distinct tendency to doubleness in the
flowers of a Ranunculus, Horse-chesnut, and Bladder-nut (_Ranunculus
repens_, _Aesculus pavia_, and _Staphylea_), growing under very
unfavourable conditions. Professor Lehman[420] found several wild
plants growing near a hot spring with double flowers. With respect to
the cause of doubleness, which arises, as we see, under widely
different circumstances, I shall presently attempt to show that the
most probable view is that unnatural conditions first give a tendency
to sterility, and that then, on the principle of compensation, as the
reproductive organs do not perform their proper functions, they either
become developed into petals, or additional petals are formed. This
view has lately been supported by Mr. Laxton,[421] who advances the
case of some common peas, which, after long-continued heavy rain,
flowered a second time, and produced double flowers.

_Seedless Fruit._--Many of our most valuable fruits, although
consisting in a homological sense of widely different organs, are
either quite sterile, or produce extremely few seeds. This is
notoriously the case with our best pears, grapes, and figs, with the
pine-apple, banana, bread-fruit, pomegranate, azarole, date-palms, and
some members of the orange-tribe. Poorer varieties of these same fruits
either habitually or occasionally yield seed.[422] Most horticulturists
look at the great size and anomalous development of the fruit as the
cause, and sterility as the result; but the opposite view, as we shall
presently see, is more probable.

_Sterility from the excessive development of the Organs of Growth or
Vegetation._--Plants which from any cause grow too luxuriantly, and
produce leaves, stems, runners, suckers, tubers, bulbs, &c., in excess,
sometimes do not flower, or if they flower do not yield seed. To make
European vegetables under the hot climate of India yield seed, it is
necessary to check their growth; and, when one-third grown, they are
taken up, and their stems and {169} tap-roots are