The technique for making one plant grow on the roots of another has been known at least since the time of Aristotle, in whose surviving writings we read: "Grafting of one on another is better in the case of trees which are similar and have the same proportions." Grafting is a useful means of accelerating growth and flowering in slow-growing species, of preserving rot-sensitive rarities and cristates. and of saving a plant that has been all but lost by rot or misadventure (6.24). It consists of bringing the cut surface of a strong root system (the stock) into intimate contact with the cut base of the desired plant (the scion) and encouraging the organic union between the two. It is much used by nurserymen, particularly on the continent of Europe and in Japan, for bringing novelties onto the market in the shortest possible time. It is mainly practised among Cactaceae, which usually unite very readily, but can also be performed with euphorbias, pachypodiums (6.25), Crassulaceae and Stapelieae. The stock and scion, as our quotation above suggests. must be more or less close relatives. For cacti, various Echinopsis, Cereus and Harrisia stocks are favourites in Europe.

and the more tender Pereskiopsis, Hylo-cereus and Myrtillocactus in warmer countries. Ceropegia tubers make a good stock for delicate Stapelieae.

Even quite tiny seedlings can be grafted, and this has been the means of preserving curious mutants of cacti lacking green chlorophyll (6.27), which would normally die as soon as the food store in the seed was used up. On a green stock they can be kept going as long as green tissue is there to feed them: as soon as it begins to cork over from old age. regraft-ing on a new stock is necessary.

Although grafting is an immense asset

to nurserymen and a means of saving the otherwise unsaveable. it can change the look of a plant if the stock is too vigorous. Botanists are therefore wary of grafted specimens, and for show purposes a plant on its own roots always has preference.

Controlling pests A glasshouse collection of succulents is little troubled by pests since the development of "systemics". A systemic insecticide is one that is absorbed through the surface of the plant, above or below ground, and remains within the cells to poison any chewing or biting c One or two good soakings of each plant annually should cope with such smaller pests as scale, mealy bug (6.26), root bug and aphis. Systemic fungicides, which deal in similar fashion with fungi, are a more recent development and offer great promise if they can lessen the incidence of

It is a mistake to place all one's faith in a single product, however, because in time resistance builds up and the cure loses its effectiveness. There is thus still room for such old-fashioned contact insecticides as nicotine and derris. A few crystals of paradichlorobenzene put in the bottom of a pot when repotting is a good deterrent against soil pests. Mala-thion preparations have proved excellent for protecting succulents, even the Cras-sulaceae, despite the makers' caution against using them on this Family. Individual brands are not named here because new preparations are continually coming onto the market. Some of the best are also

Below (6.25): The fat juicy stems of Pachypodium readily unite as gratis, as well as with the allied genus Adenium I centre). This has proved helpful lor preserving rarities.

the most poisonous, so cultivators are strongly urged to read the instructions fully, avoid direct contact with the substance. and refrain as much as possible from inhaling the fumes.

The hot, close conditions inside a propagator are favourable to the growth of rot fungi, to which some succulents (Stapelieae. for instance) are especially susceptible. Watering with mild fungicides such as Chinosol (Potassium oxy-quinoline sulphate) or Cheshunt Compound (ammonium carbonate. 11 parts; copper sulphate, two parts) takes care of these. Sciara flies are especially troublesome when composts contain much humus; the currently recommended deterrent is Diazinon.

Succulents are more susceptible than most plants to rot. It seems as if, in the dry, bright conditions of their native lands, they have built up no resistance to the cosmopolitan decay bacteria and fungi that flourish in moister, cooler conditions. At the first sign of soft black or brown patches on a succulent, all infected areas should be cut away back to sound tissue, with a sharp knife, and the plant kept dry in a light, airy place until the wounds have healed. Dusting the exposed tissue with powdered charcoal, or with a powdered fungicide such as flowers of sulphur. helps. If 9»succulent collapses from rot at the base, the stem tips can be similarly trimmed back, and treated as cuttings or grafted. Euphorbias and caudiciform plants are more difficult to save than the average cactus once rot gets into their system. Rot is encouraged by overwatering and overfeeding, by too cold or too close an atmosphere, and by anything generally adverse to growth. Some collectors regularly spray their plants with Chinosol or Cheshunt Com pound as a precaution; others place faith in systemic fungicides.

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