Sprengel, a rector in Spandau in the late I eighteenth century, was the first to study I the devices whereby flowers attract potential pollinators. He noticed the baits I offered: surplus pollen and nectar. He I observed the form and design of the I flower, and the way contrasted areas of colour (nectarguides) pointed the way to the source of food. Such guide lines in red I can be seen on the petals of Pelargonium I fulgidum (4.5). Some visitors —bats, for instance—eat not only pollen but whole I stamens. In flowers that produce no I nectar, pollen provides the sole attraction.. I
Nectar. Where nectar is developed, it is I
secreted near the centre of the bloom. I
nectaries (4.3). Certain plants also have I nectaries other than those in the flower
(extrafloral nectaries). Coryphantha and I Ferocactus species can secrete nectar I
from the areoles, where an unsightly I black mould that develops on the sugary liquid is an indication of the presence of an extrafloral nectary. Nectar is very I attractive to ants, and the extrafloral nectary may be nature's decoy to avoid I
losing the precious fluid within the I flowers to ants.
Nectar contains several substances in I
dilute solution, but the chief one is sucrose. The copious watery nectar of Aloe arborescens (13.2). a bird-pollinated species, contains 13 percent of sugar and averages 54.5mg per flower'. Bee-pollinated flowers produce less, but more concentrated, nectar; the higher the sugar content, the more the bees like it.
Scent. Sweet scents, "the swift vehicles of still sweeter thoughts" to the poet, bring delight to the gardener and bafflement to the botanist who struggles to describe them. But they are the stock-in-trade of the biochemist, who labels them as aliphatic terpene alcohols, aldehydes and so forth. And they are vital to those flowers that produce them as their main enticement, in the absence of showy display and colours—Neohenricia. for example. Not only are scents hard to describe, but human noses often disagree when comparing them. Euphorbia caput-medusae, which smells intolerably rank and musty to me, was praised as all sweetness and serenity by another grower. It has been shown that an ability to discriminate among different-coloured freesias by their scent is genetically controlled, and no amount of training or perseverance can alter heredity.
Among the most powerful fragrances in succulents (indeed, in any flower) is that of the night-flowering cerei (4.11), whose often large, white blooms of matchless purity and delicacy have justly earned them the title "Queen of the Night' ever since the day when Marie Antoinette summoned Redoute. the celebrated Belgian flower painter, to the Temple where she was kept prisoner to immortalize the bloom of her favourite Seleni-cereus at midnight, witnessed by the assembled court and royal family. The perfume varies from one species to another, those visited by bats (4.6) having a musky, acid aroma. Brought into the home, a single bloom will fill the house with a pleasing fragrance, perceptible many metresa way. but close up the effec* is overpowering, as if the nose is surfeited.
Although opinions differ over the scents of some crassulas and euphorbias, there is complete unanimity about one group, the so-called carrion flowers (4.7). These Stapelieae are the classic example of flowers adapted for attracting flies and bluebottles, and in varying degrees they simulate carrion not only in form, texture and colour but also in scent. So successful is the ruse that even in cultivation flies often lay their eggs on the corolla, although there is no food supply for the larvae and they die in consequence. Despite their "bad breath", the Stapelieae are popular with collectors for the remarkable form and diversity of their flowers, which range from less than 1cm (%in) to over 30cm (12in) in spread.
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