The families of succulents

When referring to succulents many people speak rather of ‘cacti’ and include in that name the other groups – if, indeed, they realise that there are other groups. Now, the cacti are members of the family Cactaceae; it is certainly an important one, but it is only one of the families with succulent members.

It is, of course, the flowers that reveal the family, the flower being the only part of the plant which retains its basic structure whatever adaptations the leaves and stems may have made. There are members of other families which are almost indistinguishable from cacti to the casual eye, but the flowers are entirely different when examined carefully.

The Cactaceae are almost exclusively natives of America. There are a few other succulents in America, but on the whole the other groups are African. The close resemblance of some American and some African species is a nice example of what is known scientifically as parallel evolution – the modification of members of unrelated families into similar forms and structures to meet a similar set of climatic conditions.

There is a certain amount of nomenclatural confusion among the genera. In the most important families, the Cactaceae and Aizoaceae, the first plants of each to be found were given the names, respectively, of Cactus and Mesembryanthemwn. As more and more different plants were discovered, however, it soon became plain that these names were insufficient to classify the families. For it must be remembered that names are not given merely for identification; if that were so all the Cactaceae might, indeed, be regarded as different species of a genus Cactus.

The names are also a guide to classification. Thus the families have been gradually divided into more and more genera as the botanists find details which will segregate them, and botanists differ in their ideas of what generic and specific names also may be accepted. This accounts for the various names under which some plants may be found. I have kept synonyms to the minimum, but these facts should be remembered if you see a familiar plant under another name.

The Cactaceae

A family almost entirely succulent and usually leafless. The more primitive and unadapted members are those which are least succulent and may have leaves.

The cushion-like areoles are unique to the cacti. These may be covered in wool or hair, or small bristles called glochids, and may sprout spines. It is at the areoles that the flowers usually appear; in Mammillaria, however, they do so between the tubercles. The flowers have a large, indeterminate number of petals and stamens; there is generally no clear distinction between petals and sepals. The ovary is ‘inferior’, i.e. below every other part of the flower. The flowers are often large, showy and bright. The fruit is a single-celled berry, frequently fleshy but sometimes dry, usually brightly coloured, with many seeds.

The Cactaceae are divided into three tribes, as follows.

The Pereskieae A small group of primitive cacti which are hardly succulent at all. They are mainly twiggy bushes or sub-shrubs, bearing spines and glossy leaves. The main genus is Pereskia, and most of the species are Mexican in origin.

The Opuntieae An extensive tribe containing the well-known Prickly Pears. Botanically they are separated from other cacti by the presence of glochids in the areoles – fine, barbed bristles which can be very irritating if they enter the skin. Some have awl-shaped leaves which are often only carried for a short time, and the stems are divided into more or less regular, often flattened joints. The main genus, Opuntia, contains many species, is spread from Canada to Cape Horn, and has been naturalised in the Mediterranean area, Australia and elsewhere.

The Cereeae

This tribe contains the majority of cacti, and its members are found from Canada to Patagonia. Owing to its size, it is much subdivided.

The Aizoaceae

This is a family (sometimes called Ficoidaceae or Mesembryanthemaceae) exclusively composed of succulents, from S. and S.W. Africa. The species used to be classed under the single genus Mesembryanthemum, but this has been greatly divided up, and the name now only refers to a few annual and biennial species which make good bedding plants. The flowers of the Aizoaceae are bi-sexual, with either many petals or none, 4 or 5 sepals, 4, 5 or many stamens, and an ovary of 2 or more cells giving rise to a capsular fruit. Flowers are large and bright in a wide range of colours, and superficially resemble daisies.

The plants exhibit gradations of form according to the amount of specialisation imposed by the habitat. First there are shrubby, more or less woody plants, with leaves arranged in well-spaced opposite pairs, one pair at right angles to the next. These are usually easy to grow, flower freely and are often used in gardens. They include species of Carpobrotus, Delosperma, Drosanthemum, Lampranthus and Oscularia.

Then there are plants with fairly long stems but much fleshier leaves, more closely packed on the stems. These merge into the really fleshy forms, where the stems are almost or entirely absent, often with only a few pairs of leaves, still in the cruciate pattern which is, in such forms, much more marked, though sometimes obscured by crowding. Among the genera here are Argyroderma, Cheiridopsis, Faucaria, Gibbaeum, Glottiphyllum, Pleios-pilos and Stomatium.

The next stage comes when these leaves are reduced to two; there are forms in which the leaves are partly joined, and these lead into those in which the leaves have been converted into a single solid mass or plant-body. The division between the pair is sometimes marked by a groove or slit, may be reduced to a small aperture or may have disappeared. Notable genera in this group are Conophytum, Lithops and Ophthalmophyllum.

These are most surprising when they produce their flowers, which are often bigger than the bodies themselves. At other times many of these succulents are very difficult to see, since their shape and colouring are so similar to surrounding stones. Their nickname is ‘living stones’ or ‘stone mimics’.

Apart from these are the window plants, for example Fenestraria, which might be taken for plant-bodies, but which are in fact groups of cylindrical leaves arising from one stem.

Lastly there is a group of annuals, small branching plants, two of which, Dorotheanthus and Mesembryanthemum crystallinum, are commonly treated as half-hardy annuals and can be very showy.

The Crassulaceae

This family of succulent plants includes many hardy species. The leaves are usually either in opposite pairs at right angles, in rosettes or arranged spirally up the stem. The flowers, usually carried in a cluster, are bi-sexual and regular, with 4 or 5 petals and the same number of sepals.

The names of the genera have been very much muddled, and one species may have three or four generic synonyms. One group is centred on the hardy Sempervivum, and in-cludes the tender Aeonium and Greenovia. Sedum, the stonecrops, is another genus which is fairly clearly defined.

The other important genera are Adromischus, Bryophyllum, Cotyledon, Crassula, Echeveria, Kalancho’e, Pachyphytum and Rochea.

Many of these are very attractive, and almost all are easy to grow, apart from some very specialised desert crassulas. They are widely distributed in America, Africa and to a lesser extent Asia, and show succulent forms provoked not only by heat and drought but by cold and drought.

The Asclepiadaceae Family of Succulents

This family is far from exclusively succulent. The succulent genera are on the whole African, and there are some from India and the East Indies. Most of them are leafless and superficially cactus-like, the genus Stapelia being the representative example.

The Asclepiadaceae have bizarre flowers, reaching the height of oddity in Ceropegia. Botanically they are regular; the anthers are joined to the stigma; and the seeds, which are produced in a pair of long horn-like or podlike follicles, are usually tufted with hair, a dispersal device.

The Compositae

It would be surprising if the largest of all plant families had not produced some succulent forms. The huge genus Senecio, which includes our native ragwort and groundsel, contains a number of diverse succulent forms exhibiting different stages of adaptation to drought. This genus now includes Kleinia which contains species equally diverse in form. All have typical composite flowers, disconcertingly like those of groundsel. Most of the species come from South and South-west Africa; others from other parts of Africa and from the East Indies.

The Euphorbiaceae

One of the most extensive single genera, Euphorbia contains a large number of succu-lent forms, mostly very cactus-like, but distinguishable partly by the entirely dissimilar flowers and by usually having milky juice. The genus displays the most astonishing diversity of form and adaptation. Some species are woody or herbaceous with normal leaves, while others have reduced leaves, and stems which are succulent and cactiform. The flowers are of two sexes, usually carried in cyathia, in which a petal-less female flower is surrounded by brighter petal-like structures. Sometimes the male and female flowers are carried on separate plants. Very often leaves are absent or primitive.

The succulent euphorbias occur in South and North Africa, Madagascar, Canary Islands, Arabia, E. Indies, a few in America, and are naturalised elsewhere

The Liliaceae

The succulent Liliaceae are mainly rosette plants, the leaves being often, as in Aloe – one of the most widely spread African succulents — held at the end of a woody stem. Other genera include Apicra, Gasteria (in which the leaves are carried in two rows) and Haworthia, which contains a variety of small rosette plants including several ‘window’ plants.

The flowers of these plants are like trumpet lilies in miniature, carried on long stems, but often insignificant.

Less important families

Agavaceae Includes the extensive American genus Agave, formerly placed in Amary-llidaceae. This genus has been naturalised also in Europe and Africa.

Geraniaceae The common pelargoniums so much used for bedding in some countries are themselves more or less fleshy; and there are several really succulent members of this genus from arid regions of South and South-west Africa, as well as the entirely leafless, greatly drought-resisting Sarcocaulon (rarely grown), from the same area.

Portulacaceae This family, represented in Britain by purslane and other weedy plants, is naturally fleshy, and it is difficult to draw the line as to which are succulents and which are not. One species, the Brazilian Portulaca grandiflora, is familiar as the parent of a race of showy half-hardy annuals.

Vitaceae The vine family is represented by a number of African species of Cissus (a genus mostly of climbers) with greatly swollen stems from which, in the growing season, large fleshv leaves are nroduced.

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