(iv.) Class B' is an extension of Class B and includes the flowers of the Compositae, most Dipsaceae and some Campanu laceae, in which the flowers have the same length of tube, etc., as Class B, but are aggregated into an inflorescence, which by the uninstructed might be mistaken for a flower, and which acts like a single flower as a unit of attraction.
(v.) Class F. Lepidoptera Flowers.—This includes those flowers in which the floral tube has been so deepened that short-tongued insects are excluded altogether and in many cases only Lepidoptera can reach the nectar. The alpine moss campion (Silene acaulis), for example, is adapted to butterflies, while the bladder campion (S. inflata) is adapted to moths and emits a scent at night. To the latter class belong also the honeysuckle (Lonicera), tobacco plant (Nicotiana), evening primrose (Oenothera), and night-scented stock and many others.
(vi.) Class H. Bee Flowers, are those which are visited mostly by long-tongued bees, the depth of the tube being 6 to 15 mm.
The flowers are also often markedly zygomorphic (i.e., having a special kind of irregularity; see FLOWER), providing a landing place for the bee ; others are of such a shape that (as in the snap dragon and broom) it requires an insect like the humble-bee which is not only "clever" but of considerable size and weight in order to open the flower.
(vii. and viii.) The D and K classes of flowers include those adapted to small insects; they are pollinated by flies, beetles and small bees.
Lastly, there is (ix.) the Class Po, Pollen Flowers. These pro vide no nectar, but abundant pollen for which the flower is visited, mainly by bees ; examples are Clematis, meadowsweet (Spirea), rock rose (Helianthemum) dog rose (Rosa caning), poppy. In some flowers, such as Cassia, some stamens provide "food-pollen" for insects, other stamens supply the fertile pollen for fertilization.
In some cases the nectar is secreted by one organ and collected in another as in Viola, where it collects in the spur but is secreted by appendages of the stamens. In other cases there is no free nectar but the insect must pierce with its proboscis the juicy, sapid cells, which in the case of some orchids line the spur.
The importance of colour in attracting the attention of insects is obvious, but it does not follow that the flowers most striking or attractive to our eyes are those most conspicuous or alluring to the insect. The problem of the colour-sense of insects has been in vestigated by a number of workers and it seems clear that bees at least can distinguish some colours, such as blue and yellow, and do not merely depend on the different brightness between, say, a deep purple flower and a light yellow one. In the different flower classes (A to H) already defined, there is a tendency to a progres.
sion in colour; the simpler flowers in the A and AB classes tend to be white or yellow, while in the B class blues and purples are found. In class H, the bee flowers, blues and purples predominate, while in class F, the butterfly and moth flowers, pale tints of pink and purple are most common. The special colour markings on flowers, such as the yellow eye of forget-me-not (Myosotis), the darker lines on the petals of the violet and pansy, appear to assist the insect to find the nectar and are known as honey guides.
Scent is obviously of great importance and the olfactory sense of some insects such as moths is very much greater than ours.
J. H. Fabre showed that moths when out of sight of honeysuckle would fly straight to it from a distance of several hundred yards.
K. van Frisch has investigated elaborately the olfactory sense of the honey bee. Bees which had been drilled by association of oil of orange with sugar could pick out this scent from 43 other ethereal oils. As stated by M. Skene (see p. 172) the general con clusion is "that colour is the guide to the flower, and that scent is useful in enabling the bee, flying among the many flowers of similar colour, to pick out the species it has formed the temporary habit of visiting." In this it is helped by the sense of form.