As it is obvious that with plants in which the sexes are
separate, pollen must be carried by some means from flower to flower,
such species are excluded from the lists; as are likewise dimorphic and
trimorphic plants, in which the same necessity occurs to a limited
extent. Experience has proved to me that, independently of the exclusion
of insects, the seed-bearing power of a plant is not lessened by
covering it while in flower under a thin net supported on a frame; and
this might indeed have been inferred from the consideration of the two
following lists, as they include a considerable number of species
belonging to the same genera, some of which are quite sterile and others
quite fertile when protected by a net from the access of insects.
[LIST OF PLANTS WHICH, WHEN INSECTS ARE EXCLUDED, ARE EITHER QUITE
STERILE, OR PRODUCE, AS FAR AS I COULD JUDGE, LESS THAN HALF THE NUMBER
OF SEEDS PRODUCED BY UNPROTECTED PLANTS.
Passiflora alata, racemosa, coerulea, edulis, laurifolia, and some
individuals of P. quadrangularis (Passifloraceae), are quite sterile
under these conditions: see 'Variation of Animals and Plants under
Domestication' chapter 17 2nd edition volume 2 page 118.
Viola canina (Violaceae).--Perfect flowers quite sterile unless
fertilised by bees, or artificially fertilised.
Viola tricolor.--Sets very few and poor capsules.
Reseda odorata (Resedaceae).
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