516 THE FERTILISATION OF FLOWERS. [PART III. 
REVIEW OF THE SPECIES OF POLYGONUM. 
The species of Polygonum, like those of Geranium, show clearly 
how with conspicuousness and abundance of honey the number 
and variety of visitors increase, and how, as the likelihood of cross- 
fertilisation is thus increased, the possibility of self-fertilisation 
becomes less important; the converse is also clear. But they 
show at the same time that the abundance of a plant is in no 
way determined alone by the certainty of cross-fertilisation. 
P. fagopyrum and P. Bistorta have the most conspicuous 
flowers, the most abundant honey, and the most numerous visitors ; 
in both, cross-fertilisation is insured and self-fertilisation rendered 
difficult or impossible-—in the former species by dimorphism, in 
the latter by well-marked proterandrous dichogamy. P. Persicaria 
and P. lapathifoliwum have much smaller flowers and scantier 
honey ; but, by union of their flowers in a spike, they become fairly 
conspicuous and attract fairly numerous visitors; both waver 
between insuring cross-fertilisation and insuring self-fertilisation, 
and seem to make use, toa great extent, of both processes. In 
P. minus the flowers are about as large and rich in honey as those 
of P. Persicaria, but are united in much looser, thinner spikes; 
they are therefore less conspicuous, less visited by insects, and 
more subject to self-fertilisation. Finally, in P. aviculare, whose 
flowers are not only much smaller, but are solitary and devoid or 
almost devoid of honey, insect-visits and consequent cross-fertilisa- 
tion are only the exception ; yet this species is with us the most 
abundant of its genus, and one of the commonest of our native 
plants. 
The species of Rumex are anemophilous, and Axell figures the 
flowers of Rumex crispus to compare them with the entomophi- 
lous flowers of Rhewm Rhaponticum ; I have however found a bee, 
Halictus cylindricus, F. 2, busily engaged upon the anthers of 
Rumex obtusifolius, L.; and in the Alps I have often seen red butter- 
flies seated on Rumex when in seed and like them in colour (590, 
II. ; 609). 
Orv. CYTINACEZ. 
Brugmansia Zippelii, Blume, is fertilised, in Delpino’s opinion, 
by carrion-flies which are detained for a time in the flowers (178). 
