VOLUME 14 :: January—October, 1952 Illustrations shown in ...

THE CACTUS AND SUCCULENT January, 1952
THE SCIENTIFIC APPROACH TO SUCCULENTS
By GORDON D. ROWLEY
INSTALMENT FIVE
Practical Experiments for the Amateur
January is the month for planning the year's work in the glasshouse, and as good a time as any to take the plunge
and prepare for some constructive experimenting. Here, then, are details of just a few more obvious projects.
1 Propagation problems
Finding novel ways of multiplying rare plants is a never-ending source of amusement. I recall my own surprise
and delight on receiving a robust Pelargonium echinatum grown from a snippet of a flower stalk ; on raising Pereskias
from leaves stuck in Vermiculite like Crassulas, and on watching discarded bulb scales from an old Bowiea sprout
bulbils until I had so many I didn't know where to put them all. No less strange is the way that many promising
leaves come to nothing. Some may root, but fail to grow a shoot—a common experience in Haworthias, Kleinias
and Ficoidaceae. Would treatment with rooting hormones have the desired stimulating effect, I wonder ?
Of particular interest to the propagator are those curious succulents to which I have applied the name
" caudiciform 1 ." These have in common a stout, fleshy, perennial trunk or " caudex," and slender, leafy, often
short-lived branches. Owners of rare plants like Testudinaria, Idria, Fockea, Stephania, and the tuberous Adenias
would earn our lasting gratitude if they could work out means of reproducing them from cuttings of roots or side
branches. The case of the Medusa-head Euphorbias that require two steps before side branches can be induced to
form symmetrical heads has never been satisfactorily thrashed out, and bristles with possibilities for the keen
experimenter.
2 Grafts and Chimaeras
Cutting fat plants into pieces and sticking them together again has a great appeal for many growers, and the
suggestion that this may serve useful ends should set razors stropping all over England. What happens when
a cactus is grafted upside down ? Do new shoots arise from its lower or upper end ? I cannot say : perhaps
some enterprising reader will enlighten me. How does one species react to grafting on different stocks ? In the
world of fruit trees we recognise stocks which influence vigour and form of growth. Is it true that for best results
the stock and scion should be genetically related—a Cereus on Cereus stock, Opuntia on Opuntia stock, and so on ?
Also, why among succulents is grafting almost the exclusive privilege of the Cactaceae ?
Then there are in cultivation those very strange plants that we call chimaeras. A chimaera consists of two
plants in one : a skin of one over a core of the other. In this side by side welding of tissues, each layer preserves
its identity, but character expression is dependent on a sort of internal tug-of-war between them. A break in the
skin exposes the core, and if a shoot arises here (as when plants are grown from root cuttings) it will be different
in form and all of one tissue. Often this happens naturally, or shoots may appear composed of the surface tissue
only : three different forms on one plant ! Variegated plants are often chimaeras and sort themselves out in
this way : the striped form of Aichryson domesticum, for instance, always throws some branches that are wholly
yellow. Sedum telephium variegatum when raised from root cuttings reverted to the original green form : an
example well worth following up for variegated Sedums, Agaves, and any striped succulents. If root cuttings cannot
be raised, try scooping out all the buds from the stem and forcing new shoots to break out from the interior.
Chimaeras arise in nature by mutations, but can be synthesized by grafting. If a number of grafts are cut
back leaving a thin film of scion covering the stock, a chance bud arising just at the point of union may contain
a mixture of the elements of both species. Stem succulents should prove ideal subjects for chimaera production,
being simple to graft and having a large area of union. An excellent picture of a cactus chimaera is shown by
J. J. V. Wolthuys in his book "The Enigma of the Origin of Monstrosity and Cristation in Succulent Plants "
Ed./2 (1948) 77, though the author's fanciful remarks about "vegetative splitting "of a hybrid are clearly inadmissible
in the light of the chromosome theory.
3 Fasciations
The occurrence of fasciations, nowhere more frequent or more startling than in some families of succulents,
has long puzzled botanists, and it is evident that no one theory will account for the origin of all such forms. Like
callus-formation, fasciation is a reaction against some irritant, whether that irritant is an external injury, a
nutritional upset, a pathogenic infection, or a straightforward genetic factor. Sometimes it is inherited ; sometimes
only the tendency to fasciate is passed on, needing only some shock to start it off ; sometimes neither takes place.
Undoubtedly more information is needed here, and any reader fortunate enough to have blooms on a crested plant