Form and variety

Most of the Crassulaceae are herbaceous perennials, but a few are annual (Sedum coeruleum) or biennial (S. pilosum. sem-pervivoides 10.13). The largest. Crassula arborescens (9.3) and argentea. form small trees with fat. fleshy trunks 2m (6^ft) or more in height. Almost caudici-form is Cotyledon paniculata the "botter-boom" of southern Africa, with a massive gnarled trunk and more or less deciduous leaves. It has a close parallel in the giant Mexican stonecrops Sedum frutescens and S. oxypetalum. Stem succulence takes on a different guise in Cotyledon ivallichiana, which hasa rathercactiform look from the thick branches covered with persistent leaf bases forming long slender tubercles. One or two other peculiarities of habit are to be found described in the chapter on caudiciform plants (Chapter 21).

The leaves of all the Crassulaceae are xeromorphic, although in Crassula coc-cinea. some kalanchoes and sedums only slight succulence develops. Drought resistance is not, however, proportional to the bulk of water-storing tissue. The ability of stems of the native European roseroot (Sedum rosea 10.1) and orpine or livelong (S. telephium) to stay fresh and green and even to flower long after cutting was noticed early and led to the first recognition of succulents as a group (page 12), as well as to superstitious beliefs similar to those mentioned under the account of Sempervivum (page 127).

Flowers of Crassulaceae are generally on the small side (2.19). but are usually massed in flat or domed heads which make them conspicuous to insects. Some are sweetly perfumed, others musky or foetid. Especially welcome are those African species that flower in the autumn and winter in cultivation: species of Crassula. for instance, that brighten the glasshouse when much else is dormant. The colour range is from white to pink, red, orange and yellow, with one solitary species of a beautiful pale sky blue, the annual Sedum coeruleum. The fruit is a follicle: that is, like a tiny pea pod, splitting along the inner margin to release quantities of fine, dust-like seed. The best way to collect the seeds of Crassulaceae is to cut off the inflorescences after flowering and invert them in a paper bag (not polythene, which holds the moisture too much), leaving this undisturbed on a dry, light top shelf until ready to packet or sow the seeds. It is a mistake to sow too thickly: one way to avoid this is to mix the seed with fine sifted sand or peat.

A natural Family

The Crassulaceae are a natural Family, which is to say that it is easy to recognize plants that belong to it by vegetative appearance and the regular, isomerous flowers with free carpels. The closest ally is the saxifrage Family. Saxifragaceae. but this is never truly succulent and has united carpels and other more advanced features. But. as with many natural Families, the internal classification into subfamilies and genera is unmanageable, as will be apparent even from the oversimplified synopsis given here.

A few subfamilies, such as Kalanchoi-deae and Sempervivoideae. are sharply defined and readily recognized. But Sedoi-deae is the 'dump' into which every genus that does not fit comfortably elsewhere is thrown. Uhl (1963) has done good work in attempting to clarify the position, adding data from chromosome numbers, which segregate some groups and not others. As for the genera, although they appeared distinct when first defined on a limited number of known species, many are now linked by intermediates. Consequently. some species have been shuffled

THE STONECROP AND HOUSELEEK FAMILY (CRASSULACEAE)- A breakdown to genus level

THE STONECROP AND HOUSELEEK FAMILY (CRASSULACEAE)- A breakdown to genus level

Petals united for less than halfway into a tube; inflorescence lateral, axillary

Subfamily 4

ECHEVERIOIDEAE

Flower parts usually in fives—can be 3 to 12; inflorescence usually terminal

Subfamily 5 SEDOIDEAE

Flower parts in sixes or more; petals free; inflorescence terminal

Subfamily 6

SEMPER VIVOIDEAE

Subfamily 5 SEDOIDEAE

Subfamily 6

SEMPER VI VOI DEAE

Subfamily 4

ECHEVERIOIDEAE

Dudleya

from one genus to another and may end upas sole representatives (monotypes) of a new genus to themselves. For instance, the lengthy synonymy of the plant now known as Graptopetalum pa rag ua ye rise results from its classification, at one time or another, in five different genera. Thus there are exceptions to all characters used to define the higher categories in Crassulaceae. and it is clear that convergent evolution has taken place many times over. The six subfamilies are retained in absence of better, and at least they have a geographical foundation and occupy separate, although overlapping, areas.

Intergeneric hybrids are known between species of related genera in subfamily Echeverioideae (X Craptoveria (10.2) = Graptopetalum X Echeveria. etc) and subfamily Sempervivoideae (X Greenonium = Greenovia X Aeonium, etc) as well as between subfamilies Echeverioideae and Sedoideae (X Pachy-sedum = Pachyphytum X Sedum and X Sedeveria = Sedum X Echeveria). This suggests close affinity between the genera and adds to the suspicion that they are artificially segregated.

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