tike I In- bramble Some huit no branches, others hire u ./real number, like the tyeamwe."
Stem succulence implies a greater modification of the plant body than leaf succulence, where the leaves are still the main centres of photosynthesis and the habit may differ little from that of ••normal" plants. In stem succulence the green tissue, and in consequence the vital centre for food-making, is more and more shifted to the stems (2.8)— completely so in those plants, such as columnar cacti, where leaves are undeveloped. There are many plants in which the transfer of leaf functions to stems is unaccompanied by succulence. We call them "switch plants" and familiar examples are the broom (Sarothamnus) and butcher's broom (Ruscus). They are xerophytes, surviving drought by means other than water storage. The leaf-like organs in Ruscus are actually stems. But such plants are
As with leaf succulents, five Families are singled out here for separate treatment, leaving a small residue of others that can be dealt with first in summary. All are examples of widespread, adaptable Families rich in different life forms among which are a few succulents.
This ubiquitous and bewilderingly large Family has already been mentioned under leaf succulents and makes an appearance a third time in the chapter on caudiciform plants. Although it shows extraordinary diversity in the vegetative characters, the flower headsare standard throughout and are the unifying principle whereby botanists recognize the Family. They repay a closer look, not only because of the beauty of their design and functional efficiency, but because an understanding of their structure enables one to recognize the Family at a glance and avoid referring to a head of chrysanthemum or dahlia as "a flower".
What appears at first sight to be a single bloom is seen to be made up of a number of small, separate, tubular flowers (florets) packed tightly together in a level or domed head at the end of the axis, and surrounded by a protective cup made up of many leafy bracts—an
Right (15.1). The capHulum ol Senecio stapeliiformis (left) showing the many tubular disc florets surrounded by a sleeve-like protective involucre of bracts The calyx is represented by a ring of hairs (below left) that persists as a parachute on the fruit Structure of a single Horet is shown at right For a capitulum showing both disc and ray
Below(l5.2): Senecio articulatus. thetough. easily multiplied and long-popular 'Candle Plant'. Shape ol joint and colour of markings vary according to the method of cultivation
involucre. It is thus a very condensed form of inflorescence: a capiiulum ("little head", literally), to use the official term. In Senecio stapeliiformis (15.1), all the florets are alike: tubular, with a regular five-parted corolla. We call them disc florets. In S. crassissimus (4.12) two sorts of florets are present in the one head: disc florets at the centre and. surrounding them, ray florets in which the corolla of each is pulled out on one side into a long, strap-shaped, coloured ray. Thus the resemblance to a single large flower is even more striking, each ray floret being a single "petal". Often the ray florets are unisexual or sterile.
Looking now at a typical disc floret (15.1) we see that the five stamens are united by their anthers, which form a long sleeve surrounding the central style. Initially the style is short, with closed inactive stigma lobes, and by its elongation as the flower opens it pushes out pollen from the centre of the anther tube like a piston. Look at any freshly opened flower head of the Compositae and you will see golden blobs of pollen freshly extruded from the centres of the youngest florets near the middle. After most of the pollen has been taken by insects feeding or crawling over the capitulum, the two stigma lobes spread outward and the tips become receptive. As in the cactus flower described on page 48. there is in many Compositae a further device that allows selfing if the flower head is unvisited: the wilting stigmas contact the anthers.
After detailing the niceties of a mechanism ideally suited to guarantee fertilization and seed set. preferably by outcrossing, it is unfortunate to have to admit that, although it works perfectly in habitat, seed is hardly ever set on the succulent representatives in cultivation, even when hand pollination is used. Presumably the delicate mechanism is upset by a more humid atmosphere, lower light or some other little-understood factor. Propagation therefore has to be by vegetative means alone, except when imported seed is offered.
The fruit, when set, is crowned by a brush of fine silky hairs (all that remains of a calyx), which cluster around the top of each ovary. They form a parachute that aids dispersal, as in the dandelion, a common weed of the same Family.
Of stem-succulent Compositae. an old favourite is the 'Candle Plant'. Senecio articulatus (15.2), introduced from South Africa in 1775. It is almost hardy and very easy to propagate, because the sausage-shaped joints drop off at a touch and readily strike root. The plant puts out lobed flat leaves when watered, and drops them when kept dry. It looks best in full sun; in the shade the joints elongate and lose their bright colours. A variegated cultivar is also in cultivation.
Senecio anteuphorbium occurs in
South Africa and Morocco; 5. kleinia in the Canary Islands. The variegated culti-var of the latter, called 'Candystick' (15.3), is the one to choose. S. deflersii has stems like cucumbers, and 5. stapeli-iformis lives up to its name and looks much like a Stapelia with an angled stem and scale leaves from low tubercles. These species grow upright, but S. pendulus curls over and burrows in the soil, producing a series of arching low stems—the 'Inchworm'. The last two have showy flower heads in crimson and orange-red respectively. The above species may also be met with under the generic names Kleinia and Notonia.
This Family, which is predominantly leaf-succulent, has been introduced in Chapter 12. One of its most isolated and curious members is endemic to the island of Guadalupe off the coast of Mexico: Talinum guadalupense (15.4). Caudici-form in nature, it produces long, sausage-shaped stems and becomes shrubby in captivity. The rosettes of grey, semi-persistent leaves recall Echeveria, and the delicate, purplish pink flowers are showy. It is a rare and difficult plant to keep in Europe, but seems to thrive in California.
Also worth mentioning is the genus Ceraria. curious small-leaved shrubs of South and South West Africa, with thickened stems. Young specimens have an attractive bonsai look (12.2) that is worth trying to preserve by keeping them underpotted and underwatered and trimming off any long shoots they produce. I have failed to propagate them by cuttings.
The Vitidaceae take their name from the most important genus, Vitis. the vine. Among their dozen genera and 700 species, stem succulence has evolved in a few species of Cissus, and a caudiciform habit in Cyphostemma (2.21, Chapter 21). Cissus quadrarigularis (15.5) is a wide-ranging and variable species from southern and tropical Africa and India; it produces long climbing stems that support themselves by means of tendrils. The stems are short-jointed, square in cross-section with thin, slightly wavy angles, and produce a lobed leaf at each node. The tiny flowers are followed by black "grapes", but these are not edible. The plant is worth growing as a novelty, but relishes heat: a minimum of 10°C (50°F) in winter, and more if possible.
Right (15.3) Senecio kleinia. endemic to the Canary Isles, forms a stem-succulent shrub up to 3m (I Oft) tali This is the variegated cultivar. called Candystick'. introduced by the author in 1973. Propagation is by cuttings which are rather temperamental about rooting.
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