THE FERTILISATION OF CERTAIN PLANTS. 49 
arrangement. It may indeed be objected that the evil would 
have been equally avoided by all four anthers being placed in 
one horizontal line. Of two perfectly equal arrangements, 
nature, however, can but select one. But, in fact, the horizontal 
position would not answer all the requirements. Let any one 
watch a bee visiting a woodsage. He will see that the anthers 
strike the top of its head just between the eyes, and smudge the 
whole intervening space with pollen. Had the two lower anthers 
been placed in a horizontal line with the upper ones, their 
pollen would have been shed on the eyes of the insect, and its 
visits discouraged. The pollen, moreover — and this is the point 
of chief importance — would have adhered to parts which are 
quite external to the range of a centrally placed stigma. 
If the explanation I have just given be the true one, it should 
apply not merely to woodsage, but to all those numerous other 
flowers — labiates, figworts, and the rest — which have a similar 
arrangement of their stamens. Thus all such didynamous 
plants must be supposed to require the agency of insects for 
their due fertilisation. Now that this is really the case is, I 
think, highly probable. Why otherwise are they, as a rule, fur- 
nished with sweet secretions and aromatic odours which serve 
to attract insects? Why are their four stamens so arranged 
against the walls of the corolla as to leave a free and open 
access to the nectary ? Why are all the four anthers and the 
style placed on the same side of the corolla, and in the median 
line ? Why is the dehiscent surface turned towards the path 
which leads to the nectary and away from the stigma ? Why 
does this latter so often become mature at a later period than 
the anthers? Why are there only four stamens, even when the 
corolla has five divisions, and why is the missing stamen invari- 
ably the posterior one ? Why, lastly, do we so frequently find 
some or other arrangement in these didynamous flowers, which 
seems directly calculated to render self-fertilisation a matter, to 
say the least, of difficulty ? All these questions admit of easy 
answers, on the hypothesis that didynamia are fertilised by 
insects, and are unanswerable on any other hypothesis. 
Let any one, for instance, examine a spike of foxglove, begin- 
ning with the upper or less mature flowers, and going gradually 
downwards to the more mature. He will find, first, flowers in 
which neither anthers nor stigma are ripe. Then come flowers 
in which the upper pair of anthers are ripe, the lower pair and 
stigma still immature. Then all four anthers are found ripe, 
the stigma still remaining as before. Then, lastly, come flowers 
in which the bifid stigma is open and bent forwards, but in 
which the two upper anthers have discharged all their pollen, 
while the two lower ones have not yet exhausted their store. 
Now, why is it that the upper anthers are thus before the lower 
VOL. IX. — NO. XXXI V. E 
