To the best of my knowledge no succulent is reliant on the wind for pollen transfer (anemophilous), as are the grasses and many catkin-bearing trees and palms.
Most depend on the visits of insects (entomophilous) or birds (ornithophilous), although bats (4.6) appear to be the main pollen vector in such nocturnal cacti as the giant saguaro. But we have few accounts of pollinators observed visiting succulents in the wild. Field workers are usually too intent on seeking new species to bother. Who wants to sit in the blinding sun all day beside a Ceropegia. hoping to catch sight of the one tiny fly whose visit sets a whole pod of feathery seeds? Another problem is that you need to be an expert entomologist as well as a botanist in order to identify the insect visitors, and the days of the all-round naturalist — such grand pioneers as Sprengel, Müller, Kernerand Knuth—are no more.
Although we have only a few isolated records, we can often predict the type of visitor to a flower from its form and colour, symmetry and food source. Thus Vogel2, in studying flower form in African Aloineae, contrasted Aloe ferox (with pendent, red, bird-pollinated flowers) with Aloe(Guillauminia) albiflora (with white bells pollinated by bees) and Aloe (Leptaloe) minima (with narrow-tubed. slightly oblique whitish blooms attractive to butterflies).
Bees. Of all visitors to flowers, bees are the most efficient pollinators (4.2) because of their constancy and their ability to work mechanisms such as those of a snapdragon (Antirrhinum) or an orchid. Honeybees (Apis mellifera) have been closely studied, and we now know that they can recognize and memorize patterns and shapes of flowers (is "intelligence" too strong a word?) as well as communicate information to one another. When i flowers are scarce in early spring, bees can be seen foraging for a good source of 1 pollen or nectar. In a glasshouse a bee will go from Haworthia to Crassula, then perhaps to Rebutía. But once it finds a plentiful food source it will return to the hive and alert other bees by means of a special "bee dance", giving indications of the direction and distance of the newly I discovered food source.
Bees recognize flowers by their combination of colour, patterning, shape in I three dimensions and scent, scanning the I edges first on making their approach. I Their colour vision extends to the ultra- I violet, so that some blooms that look I plain to the human eye register to a bee as I patterned in light and dark. Their scent I perception also extends beyond ours: they have been noted taking the character- I istic flight pattern that follows reaction to I scent on approaching flowers that are I odourless to the human nose. Truly they I
Right (4.6): A bat attracted to the white nocturnal bloom ola columnar cereus.
acceptable Note pollen on the bat s head I
Below (4.7): Flies on Stapelia ambigua, one ot the carrion Ilowers Eggs are commonly laid I near the centre ol the bloom, bul the larvae die I
are remarkable creatures. As taxono-mists, bees can distinguish fine shades of difference, as between two allied species of snapdragon (Antirrhinum) grown in a mixed plot, and rarely mistake one for the
In the semi-desert regions characteristic of succulent habitats, solitary bees are commoner than social bees. These tend to be more restricted in their habits, foraging on a few or even a single species, and hence limited in activity to the flowering season of that species. This synchronization can be amazingly fine: for instance, the rise in temperature needed to open the buds on a cactus may be the same as that required to awaken activity by the bees.
Butterflies and beetles. Other insects are less "intelligent" and less faithful in their attentions to a single species. Some are quite random in their visits, and thus of minimal significance for cross-pollination. Pollen robbers such as flies and beetles (4.8), which visit flowers erratically to feed but rarely pollinate them, are positively excluded from the more specialized flowers by devices that, for example, restrict access to the stamens or hide the nectar at the base of a long tube that is accessible only to an insect possessing a suitably long proboscis. The proboscis length in honeybees is up to 6mm (!4in); in bumble-bees 8-16mm (Vr %in); and in butterflies and moths up to 15cm (6in) or more. On the other hand, hoverflies have a proboscis length of only 2-4mm (Vii-lfein), although one or two Below (4.8): Primitive insects on a primitive /tower Acmaeodera beetles on a Ilower ot Ferocactus. Beetles are inefficient pollinators: they roam at random and eat the pollen.
exceptions reach 12mm ('/Sin). They feed on both nectar and pollen.
The special case of carrion flowers, pollinated by blowflies and dungflies, is discussed further below.
Birds. Like butterflies (4.9), birds respond particularly to the red range of colours, and to the presence of nectar, which needs to be in large supply to satisfy the thirst of a bird. Among succulents, the tubular, red or yellow blooms of many aloes (13.10) are typical bird flowers, although equally accessible to bees. If there is no landing stage for the bird it hovers beneath the bloom probing upwards with its bill. In their natural habitat Below (4.9): Vanessa urticae, the Small Tortoiseshell buttertly. on tlowers o/Sedum spectabile. This is commonly planted in temperate region gardens to attract buttertlies in Africa some species of sunbirds visit aloes (4.4). but when these aloes are grown in California humming-birds are equally attracted.
Flower form and function Flowers can be arranged in an evolutionary series starting with those that attract many and varied visitors up to those admitting only a single type — the "right" one to effect pollination. The former, primitive, pattern produces an abundant food supply but is wasteful of pollen. The flowers are radially symmetrical and conspicuous, with easy landing and access to the banquet. Examples among succulents are Portulaca (5.3), the Mesembryan-themaceae and the Cactaceae, all of which produce numerous stamens—in the saguaro (16.12), over 3.400 in a single bloom, the largest number on record for any plant. It is surprising to find in cacti simple, spiralled flowers with many parts at such a low level of specialization, side by side with the extraordinarily advanced vegetative habit.
Another great Family of succulents, the Crassulaceae, also has simple, radially symmetrical flowers, but although they are individually small, the blooms are massed together in flattish or domed heads (corymbs), which makes them collectively as conspicuous as a single large flower (4.10). Small and large insects find an easy landing ground, and nectar is accessible to the shortest proboscis. In biting stonecrop, Sedum acre*, the five erect outer stamens shed pollen first, then the inner five rise up and take their place, prolonging the effective life of the flower. The stigmas mature late, but sufficient pollen is still around to
Timely flowering. Large, flat, open flowers in the desert sun are a great drain on the plant's water reserves, and we find that they are usually short-lived, or open only at set hours during the day, presumably when the right pollinators are active. In 1794 Haworth4 suggested making a floral clock using different species of Mesembryanthemaceae, which, he noted, have specific times for opening and closing. The largest cactus flowers open only at night (when evaporation is much less intense) and are pollinated by moths or bats. Because pollen is spoilt if wetted, many cacti and Mesembryanthemaceae close their flowers rather rapidly when the sky clouds over—a fact soon discovered by would-be photographers of
these flowers, to their annoyance.
Another phenomenon to be seen in certain cacti and Mesembryanthemaceae is sensitive stamens. If. for instance, you touch the filaments of an Opuntia flower fully expanded on a hot sunny day. they rise up smartly and form a brush around the style, gradually returning later to the original position. This is no doubt effective in dusting the underside of a large insect visitor with pollen.
Once pollination has taken place, there would be no advantage in a flower staying open and losing precious water. In an experiment with Mammillaria flowers, those pollinated by hand were found to close two days in advance of those left unpollinated. To prolong the life of flowers in captivity, you would therefore be wise to avoid pollination unless you want seed.
Complex pollination systems. More specialized flower types are the zygomor-phic blooms of Haworthia, Coleus (9.1) and Pelargonium (4.5), which are symmetrical about one vertical plane only and tailor-made in three dimensions to fit one type of visitor—typically a bee. Below (4 101 Sedum telephium. like S. spectabile. has liny flowers, but their massing together in corymbs makes them more conspicuous and a landing stage lor a great diversity ot insects although five different pollen vectors have been reported for different species of Pelargonium'. Nectar is concealed at the bottom of a narrow tube, and the mouth of the flower has a characteristic lobing and colouring that advertise the way in, often by means of converging lines or nectar guides.
Most highly developed of all pollination mechanisms in succulents are those of the Asclepiadaceae: Stapelieae and Cero-pegieae (Brachystelma and Ceropegia). The pollen of these, instead of separating as grains, remains stuck together in a mass, one from each of the two anther cells, linked together by a yoke to form a pollinium (19.5, 7). The complete pollin-ium is transferred from one flower to another on the hairs of the leg or proboscis of a species of fly. On crawling over another flower the fly deposits the pollinium on the stigmatic surface near the centre. Recent observations on a Caral-luma in bloom in a Johannesburg garden showed that its only visitors were tiny fruit flies (Drosophila), which transferred the pollinia from flower to flower, this was the first recorded instance of a flower reliant on fruit fly as pollen vector. Right (4.1 tj Nocturnal flowers are economical in a desert they lose less water than during the heat ot the day Most are large and white with a powerful scent, as in this night-blooming cereus
In the related genus Ceropegia the corolla is developed over the essential parts of the flower into a long tube with an oddly ornate top that may resemble a lantern, canopy or umbrella (4.14). This acts as a trap for tiny flies or midges, in much the same way as does the common arum (Arum maculatum) of European hedgerows. The insects are attracted by the faint but specific scent and the garish patterns on the top of the canopy, which often includes vibratile hairs. The walls are slippery, and the insect soon finds itself trapped in the swollen belly of the corolla tube, prevented from escaping by the smooth walls or sometimes by down ward-curving hairs. Although the tube is dark, the belly is light, often from translucent "windows" that direct light onto the pollinia. In darkness the insects (tiny midges, Ceratopogon, in Ceropegia woodii) would become inactive, but in the light they buzz around, pick up pollinia and finally escape when, in a day or so.
the flower inclines from vertical to horizontal and any obstructive hairs wilt. Flies and midges have short memories and repeat the process before long, thus effecting pollination3.
Flowering communes. The Compositae and Euphorbiaceae stand apart from the succulents already reviewed in producing, not a single large flower, but an aggregation of many tiny blooms (4.12, 13) surrounded by a protective envelope so that overall the appearance is that of a single large bloom. The structure of these is further described in Chapters 9and 20. A compound head of flowers such as this has certain biological advantages over a single flower of the same bulk. The florets open serially over a long period of time, prolonging the working life. It is more or less weatherproof, and the outer covering of bracts gives added protection to the developing seeds. In a single large ovary, biting or chewing insects have only to
Above (4.12): The inllorescence ol Compositae I is made up ol two different kinds of small flowers (florets): radially symmetrical disc florets in the centre and zygomorphic ray
penetrate once and all the contents are theirs for the taking, but in a large compound head, each seed is individually protected. It is hardly surprising, then, that these two Families are among the most widespread, diverse and successful of all flowering plants.
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