146 THE FERTILISATION OF FLOWERS. [PART III, 
the amount of difference between the sexual principles that is 
requisite for full productiveness in a certain plant the greater 
i 
chance will there be — ceteris paribus—of that plant proving 
fruitful when crossed with much more distant relatives. Thus 
species in which individuals are quite sterile to their own pollen, 
and more or less unfertile to pollen from closely related indivi- 
duals, will in general be specially likely: to form hybrids with other — 
species. (Compare Abutilon, Lobelia, Passiflora, Oncidiwm) (558). 
The natural fertilisers of Abutilon at Itajahy are humming- - 
birds, which perform their work so diligently that, as we have 
seen, the power of reproduction on self-fertilisation has been 
dispensed with. 
Pavonia hastata, Cav., has cleistogamic flowers without 
nectaries (318). 
Goethea coccinea, according to Delpino, is proterogynous with 
long-lived stigmas. The tetraphyllous involucre renders the 
flower conspicuous; the honey is secreted in five glands at the 
base of the urceolate calyx, and is sheltered by the corolla. 
Delpino supposes bees or humming-birds to be the fertilisers 
(177, 351). | 
Orv. STERCULIACE. 
Melochia parvifolia, H. B. and K. (Caracas), is, according to 
Ernst, dimorphic and heterostyled (225). 
Orv. TILIACE ZL. 
73. TILIA EUROPHA, L.—Sprengel has fully described the 
flower of the lime: its proterandrous condition, which he 
overlooked, was noted by Hildebrand (356). 
The honey is secreted and lodged in the hollow sepals, and 
is accessible to insects with short proboscides. The sepals and 
petals are overtopped by the stamens, which are numerous and 
curved outwards; and insects can only alight on the anthers, or 
on the stigmas, or in the space between them. The possibility 
of self-fertilisation is almost excluded by the stamens remaining 
bent outwards to the last, while the pistil occupies the axis of 
the flower; only rarely is a flower met with in which an anther 
has become curved inwards to touch the stigma. The lime, which 
rarely produces seed in England, attracts great numbers of insects 
