Broadly speaking, there are three types of succulents, according to where the water storage tissue is located in the plant (2.9). Each represents a different life form — that is, the overall habit of the plant as determined by its ecological requirements. There is some overlap between the three groups.
Leaf succulents are plants with enlarged, fleshy, water-storing leaves, as distinct from those where the bulk of storage is concentrated in stems or roots.
Stem succulents are those in which the fleshy tissue is developed mainly in the stems, which are green (at least when young) and are the main photosynthetic centres in the plant. Leaves are often absent, or very small, flat orafrl-shaped, and deciduous; only rarely are they persistent and somewhat fleshy. Many succulents in this group have the stems conspicuously ribbed or covered in tubercles, and commonly they may be constricted into joints.
(inflorescences)—and the perennial, non-photosynthesizing storage organ at or below ground level. Further distinguish-
is the fact that the caudex is almost never green or regularly jointed, ribbed or tuberculate. Bowiea, a curious plant of the lily Family whose bulb goes green if planted above ground, is included here for convenience also.
The growth of a seedling The tough, black seed coat splits, cells divide, water swells the contents, and a new seedling breaks forth into the world The miracle of life resurgent is so familiar that we tend to take it for granted. And succulents are no different from other plants in the conditions they need for germination or the stages whereby a mature specimen unfolds from an undifferentiated blob of cells (6.19).
The primary root of a seedling heads downward in response to gravity and moist soil beneath; the stem thrusts upward to the light. The first appearance
Caudiciform ("root") succulents form the third group. 'Root' is included in inverted commas because the swollen storage organ may be all root, all stem, or root below and stem above. Hence a better term is caudex (adj. caudiciform), a neutral word for any massive basal storage organ of whatever origin. This distinctive life form is characterized by a division of labour between the shortlived, aerial photosynthesizing organs-thin leaves, shoots or flowering stems
difference is that roots never bear buds or leaves a long their length as do stems. The first root developing from a seed is called the radicle; if this continues strongly in a downward direction it is called a taproot and its side branches are laterals. If thin, radiating laterals predominate and the taproot remains short, the root system is said to be fibrous, as in most species of the Familes Crassulaceae and Mesem-bryanthemaceae. In Peniocereus and Fockea (21.2) the taproot becomes greatly expanded and may weigh several kilograms when full grown, whereas in Wilcoxia it is the laterals that enlarge to form a dahlia-like cluster of tubers. A swollen conical taproot is common to many dwarf globular succulents (2.3) and the astute gardener growing such plants allows extra sharp drainage, knowing that the soft bulky tissue is prone to rot if the soil becomes waterlogged.
Old roots develop a central woody core, which gives them great strength longitudinally while allowing them to flex a little as the trunk is rocked in the wind. In addition to providing anchorage, roots absorb water and food materials in dilute solution. Storage of water, or elaborated food, or both, occurs in roots that may become enlarged and succulent. Absorption from the soil takes place through the root hairs—special elongated cells from the surface—which are short-lived and continuously replaced as the roots grow. During periods of drought, a succulent can to some extent reverse the flow of water up the stem and transfer some downwards to keep the roots, which have no xeromorphic features, from dying back too much. Even so, conditions in nature are never as severe as in a small pot in a glasshouse left in the burning sun. where there is no large volume of soil below to buffer against rapid water loss, and no cool rock beneath which the roots can insinuate, as in the wild. Many plant losses in cultivation are due to ignorance of this big difference.
In general, roots are much more uniform and stereotyped than the other parts of a plant. If you were to plant a mixed collection of succulents upside down, with the stems concealed from view, they would look much alike and not obviously different from any other sort of plant. Because of this, roots are normally described in textbooks or cited in classifications only where they offer some rare distinction, such as the fleshy laterals of some haworthias. or the sausage-shaped tubers of Senecio hallianus.
Roots that arise from stems, leaves or aerial parts of the plant, not from existing roots, are called adventitious. All cuttings grow by means of adventitiously developed roots, and may or may not eventually duplicate the same peculiarities (as tubers, for instance) of the parent plant.
Above (2.3): Enlarged conical taproot olthe peyote cactus, Lophophora williamsii: an underground food and water store Plants with soft, fleshy underground organs need extra sharp drainage and care with watering
Below (2.4): A section ot the woody skeleton ot a cactus showing the hollow cylinder and open network of woody fibres A similar anatomy has been independently evolved in Euphorbia, Fouquieria and other succulents
When repotting any succulent with thick branching roots or tubers, it is tempting to cut off pieces and see if they can be induced to generate new plants. Sometimes they do, sometimes they don't. I have found Pachypodium succulentum one of the easiest to propagate this way. Any piece of root down to pencil thickness can be planted like an ordinary cutting.
Further considerations of root forms will be found in the introduction to caudi-ciform plants. Chapter 21.
Succulent stems Whereas a leaf succulent, on the one hand, bears thin, woody, leafy shoots, a leafless stem succulent has photosynthesis and the other functions of leaves transferred to the green stems (2.1). Between the two extremes there are all manner of intermediates (2.9), showing different degrees of leaf reduction and location of water storage. In stem succulents such as cacti, stapelias and euphorbias. normal cork formation takes place as the stems age. so that the green tissue is confined to the youngest shoots. This is a natural process and cannot be halted, although it worries some growers, who resent seeing a favourite show specimen age visibly. Ultimately the succulent
Beta* (2.5): Cross-section of Mammillaria bocasana. The central pith contains purple betacyanm and is surrounded by a ring ot conducting tissue with lurther water storage cells between that and the tough er.
tissue is largely replaced by a woody core, so that the trunk of veteran cacti and euphorbias becomes externally indistinguishable from that of normal trees. A hollow, net-like cylinder is the characteristic form of the wood (2.4): light and open in structure and mechanically ideal for supporting the weight —up to several tons—of the crown of water-filled branches.
This sort of woody skeleton has evolved independently in cacti, euphorbias, Fou-quieria and other tree-like succulents. The wood is not very exciting to the carpenter or cabinet maker, except for producing trinkets and novelties for souvenir hunters, but it is locally used for fencing and simple kinds of building.
Wherever a stem is thickly cylindrical or approaches a sphere in shape, we find that the surface is folded into ribs or broken up into protuberances (tubercles). This permits expansion and contraction without damage. Thus, in some of the giant cacti of Arizona it has been shown1 that the bulk of the plant alters considerably with water content, swelling after rain and shrinking during drought. The volume change is accommodated by con-certina-like expansion and contraction of the ribs (2.1). Without them, the tissues would split open. This does actually happen to some stem succulents in cultivation, if grossly overwatered.
Epiphyllum. Schlumbergera and related cacti, and certain euphorbias, have flat, leafless, green shoots that look and function like leaves (2.8). Non-botanists may call them "leaves", but the technical name for a flattened stem is "phylloclade", or "cladode". Their anatomy, and the presence of buds that grow into flowers or further phy Hoc lades, leave no doubt as to the true nature of such organs.
As I have suggested, the line of separation between leaf and stem succulents is often hard to draw. Similarly there is a gradual transition from stem to caudici-form succulents where the photosynthe-sizing organs are usually shed as leaves or whole shoots during the resting season and the main food and water store is at or below ground level.
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