THE COLOURS OF FLOWERS 
103 
selves, but they have yet to be proved to be adaptations for 
this purpose; they may be phenomena accompanying changes 
of colour in flowers which have been retained as useful 
characters \ 
Anthesis, Protection , &c. The bud gradually increases 
in size to a certain point, after which, given the necessary 
warmth, turgidity, &c., it opens, or anthesis occurs. The 
period during which the flower remains open is usually 
roughly constant for the same species within certain limits, 
but different species differ enormously in this respect. 
Flowers usually wither as soon as fertilisation (or even pol- 
lination) occurs ; flowers like those of orchids, in which auto- 
gamy is impossible, may remain open a very long time if not 
pollinated. 
If the time of opening of the flowers of any species be watched, it 
will be observed that a few only are open at first, then the number 
increases to a maximum, and gradually decreases again. In the flower- 
ing of a whole genus, more than one maximum is very commonly 
observed {cf p. 22). In a single district the flowering of any ecological 
class, e.g. A or H, tends to show a single period of greatest vigour, and 
this is often found to correspond approximately with the time of greatest 
abundance of the most suitable insects, e.g. short-tongued flies for class 
A, bees for H. 
The bulk of the flowers of the North temperate zone open in summer 
(maximum in July), but there are many that flower in autumn or early 
in spring. It is an advantage to plants to flower at different periods, 
especially if they are nearly allied, for the competition for insect-visits 
is thereby lessened. It must not however be assumed that the phe- 
nomenon is an adaptation to this end ; many factors influence the 
flowering of a plant, e.g. a check to the vegetative growth. A richly 
fed plant tends to vegetation rather than to flowering, and the signi- 
ficance of pruning, &c. rests largely on this fact. Plants that are to 
flower in early spring in temperate climates must have flower-buds laid 
down the preceding year and also a store of reserve food-materials with 
which to start growth. Such are Rhododendron, Eranthis, most of 
our trees, &c. The buds are usually protected by outer scaly leaves, 
hairs, or other arrangements. A few autumn-flowering plants show 
similar protections, e.g. Hedera, Hamamelis, &c., hence it is possible 
that these were once spring-flowerers, but have gradually taken to 
developing their buds more early. 
In these spring-flowering plants we have cases, otherwise rare, of 
protection of flower-buds such as is seen in winter buds of leaves. 
Xerophytes (see Chap. III.) often show protection of flower-buds 
1 Hildebrand, Die Farben d. Bliithen , Leipzig, 1879, Muller’s 
Fertilisation of Flowers , Wallace’s Darwinism , &c. 
