to refer to plants of this kind in the following chapters, I will here give a
few details respecting them. It is familiar to every one that the ray-florets of
the Compositae often differ remarkably from the others; and so it is with the
outer flowers of many Umbelliferae, some Cruciferae and a few other families.
Several species of Hydrangea and Viburnum offer striking instances of the same
fact. The Rubiaceous genus Mussaenda presents a very curious appearance from
some of the flowers having the tip of one of the sepals developed into a large
petal-like expansion, coloured either white or purple. The outer flowers in
several Acanthaceous genera are large and conspicuous but sterile; the next in
order are smaller, open, moderately fertile and capable of cross-fertilisation;
whilst the central ones are cleistogamic, being still smaller, closed and highly
fertile; so that here the inflorescence consists of three kinds of flowers.
(Introduction/5. J. Scott 'Journal of Botany' London new series volume 1 1872
pages 161-164.) From what we know in other cases of the use of the corolla,
coloured bracteae, etc., and from what H. Muller has observed on the frequency
of the visits of insects to the flower-heads of the Umbelliferae and Compositae
being largely determined by their conspicuousness, there can be no doubt that
the increased size of the corolla of the outer flowers, the inner ones being in
all the above cases small, serves to attract insects. (Introduction/6. 'Die
Befruchtung der Blumen' pages 108, 412.) The result is that cross-fertilisation
is thus favoured. Most flowers wither soon after being fertilised, but
Hildebrand states that the ray-florets of the Compositae last for a long time,
until all those on the disc are impregnated; and this clearly shows the use of
the former. (Introduction/7. See his interesting memoir 'Ueber die
Geschlechtsverhaltniss bei den Compositen' 1869 page 92.) The ray-florets,
however, are of service in another and very different manner, namely, by folding
inwards at night and during cold rainy weather, so as to protect the florets of
the disc. (Introduction/8. Kerner clearly shows that this is the case: 'Die
Schutzmittel des Pollens' 1873 page 28.) Moreover they often contain matter
which is excessively poisonous to insects, as may be seen in the use of flea-