THE FERTILISATION OF CERTAIN PLANTS. 
51 
but that of the missing stamen in which the style can be 
placed. 
The best thing, therefore, that can happen is that this stamen 
shall not be developed at all ; and this is what does occur in 
didynamous flowers. The next best thing is that, when de- 
veloped, it shall get out of the way and make place for the 
style. This is what occurs in Pentstemon. Here four of the 
stamens and the style are arranged as in ordinary didynamous 
flowers. The fifth stamen, however, is present, but makes place 
for the style, by running from its insertion in the posterior 
median line right across the tube to the opposite side of the 
corolla. It is less in the way here than if it occupied its natural 
position. Still it is perfectly useless, and in some degree an 
obstruction. It is not therefore to be wondered at that, as a 
rule, it produces no pollen, and that not very rarely it is alto- 
gether absent. The Pentstemon would thus appear to be on 
its way to become didynamous. 
Doubtless there are not a few didynamous flowers the struc- 
ture of which may seem to be more or less in contradiction 
with the preceding remarks. The apparent contradictions are, 
however, I think, always capable of explanation. Some of 
these exceptions to the ordinary arrangement I will now 
consider. 
Scrofularia . — In this genus the posterior stamen is still the 
missing one, although the style and other stamens are ranged 
against the anterior wall. The absence of the fifth stamen 
cannot therefore in this case be explained by its place being 
required for the style. It can, however, be accounted for on 
another ground. The stamen would be perfectly useless, for 
its anther would strike an insect on the back, while the stigma 
only comes into contact with the under surface. The anther is 
absent therefore on grounds of economy. But as its entire dis- 
appearance is not required to make room for the style, it is not 
surprising to find that it is not so completely absent as in oth?r 
didynamia, but is represented by a scaly staminode of con- 
siderable size. 
Gesneria . — Here we have a corolla in general form like that 
of a foxglove. The style also with its stigma occupies a like 
position as in that flower; that is, it runs in close contact 
with the posterior wall and in the median line, occupying 
the place of the absent stamen. The anthers of the four re- 
maining stamens, instead of being arranged in pairs longitudi- 
nally, as in foxglove and most didynamia, lie in a horizontal 
line immediately in front of the stigma. In the case of Teu- 
crium, I said that such an arrangement would not answer the 
requirements, because the two external anthers would smeaj 
their pollen on parts of the bee which would be quite out of 
E 2 
