6 INTRODUCTION 
intolerable ; but it is always agreeable to the insects for which the nectar is destined. 
The corolla is, except in a very few species, coloured, i.e. other than green, so that 
it is conspicuous against the green colour of the plants. Sometimes the calyx also 
is coloured, and when the corolla is developed, the calyx may be different from it, or 
if it makes one whole with this, it is similarly coloured on the inner side. But if the 
corolla is absent, then the calyx takes its place. . .. If now an insect, attracted by the 
beauty of the corolla, or by an agreeable odour, has gone to a flower, it will either 
forthwith perceive the nectar, or, if this is in a concealed place, will not perceive it. 
In the latter case Nature comes to the rescue with the nectar guide. This consists 
of one or several spots, lines, dots, or markings of another colour than that of the 
corolla as a whole, and consequently conspicuous against its lighter or darker 
tint. It is always placed just where the insects must creep in if they are to 
reach the nectar.’ 
‘In connection with the nectar guide I must refer to the difference in nectar 
flowers with regard to the time of day at which they open. As there are insects 
that only move about in the daytime, so there are day flowers and night flowers. 
The day flowers burst forth into bloom in the morning. Many of them close in 
the evening, or incline downwards, while they stand erect by day. The day flowers 
are adorned with nectar marks, though not in all cases. The night flowers 
blossom in the evening. In the daytime most of them are closed or limp and 
inconspicuous, from which it is clear that they are not destined for day insects. 
The night flowers have a large and bright-coloured corolla, so that they are 
conspicuous to the eyes of insects in the darkness of the night. If their corolla 
is inconspicuous, the defect is made good by a powerful odour. No nectar 
guides occur in them; for if the white corolla of a night flower had a nectar 
guide of another, but still light tint, this would not be conspicuous against the 
colour of the corolla in the darkness of the night, and so would be useless; 
while if it had a dark-coloured nectar guide, this would be inconspicuous, and 
would therefore be as useless as the other.’ 
5. Pollination of nectar flowers by insects: Dichogamy. ‘All these 
arrangements are in the first place and immediately for the benefit of insects, but 
through these also for the flowers themselves; and their final purpose is that the 
flowers may be pollinated by insects. That insects play their part in the pollination 
of flowers has already been remarked by others. So far as I know, KoOlreuter 
has gone furthest in this direction, as he discovered and clearly demonstrated 
the fact in /r7s, for instance, and a few other genera. No one, however, has yet 
shown that the whole structure of nectar flowers points to this purpose, and can 
be fully explained with reference to it, for no one has recognized what I call the 
nectar cover and the nectar guide to be what they are, though every one has seen 
them. ... There is undeniable evidence of the pollination of flowers by insects in 
the arrangement discovered by me in very many hermaphrodite flowers, which 
secures that no individual may be fertilized by its own pollen, but only by pollen 
from another individual. . .. This arrangement I call the development of sexual parts 
(anther and stigma) at different times, or shortly, dichogamy. It consists in this: 
After the flower has opened, the filaments have or assume, either all together or 
one after the other, a definite position, in which their anthers open, and their pollen 
