4.84 THE FERTILISATION OF FLOWERS. [PART IIT. 
this plant is very abundantly visited by humming-birds, and is 
adapted for them in the following characters : the small under lip, 
scarcely fit for a standing-place, the very large amount of honey, 
and the bright scarlet colour of the calyx and corolla (735). 
349. NEPETA GLECHOMA, Benth. (Glechoma hederacea, L.)— 
In the small-flowered female form, the tube is 64 to 8 mm. long, 
widening in the anterior half to a diameter of only 14 to 23 mm. 
laterally, and rather less antero-posteriorly : in the large-flowered 
hermaphrodite plants the tube is 9 to 16, usually 14 to 16 mm. 
long, and, for the greater part of the anterior half, 25 to 4¢ 
mm. broad, being slightly less in the antero-posterior diameter. 
The honey in the small female flowers is accessible to all our 
humble-bees, while the large, hermaphrodite flowers, by widening 
anteriorly to admit the bee’s head, render their honey accessible to 
all except B. terrestris, L. 
The existence of plants bearing only small female flowers in 
Nepeta and also in Origanum, Thymus, Mentha, and others, is not 
to be explained merely by the necessity of insuring cross-fertilisa- 
tion ; for in all these genera cross-fertilisation is completely pro- 
vided for by the hermaphrodite forms alone. But the following — 
view of the small-flowered female plants is intelligible :— | 
Of the flowers of the same species growing together, the most — 
conspicuous are first visited by insects, and if the flowers on some © 
plants are smaller than on others, perhaps owing to scanty nourish- — 
ment, they will generally be visited last. If the plant is so much | 
visited by insects that cross-fertilisation is fully insured by means — 
of proterandrous dichogamy, and self-fertilisation is thus rendered 
quite needless, then the stamens of the last-visited small-flowered 
plants are useless, and Natural Selection will tend to make 
them disappear because the loss of useless organs is manifestly 
advantageous for every organism. 
This explanation rests upon the hypotheses, (1) that the flowers 
of those species in which small-flowered female plants occur together 
with large-flowered hermaphrodite plants are plentifully visited 
by insects and are markedly proterandrous ; (2) that variation 
in size of the flowers has always taken place, not among the 
flowers on a single plant, but between the flowers on different — 
individuals, 7 ‘ 
Both hypotheses are well founded. For (1) the flowers of 
Nepeta, Thymus, Origanum, and Mentha are plentifully visited — 
by insects and are markedly proterandrous, while proterandry has_ 
