The Gymnocalycium group which is illustrated above and on the following two pages is one of the most rewarding genera for the amateur, as most species flower reliably and continuously over a long period. They will benefit from being placed out on the rock garden during the summer if space in the greenhouse is at a premium and. at any event, should be given an airy position on a shelf or near the door of the greenhouse as this' will help to ripen the areoles. The presence of flowers can be detected fairly early on during the growing season when small buds appear in the areoles just above the spines. These appear at first on those nearer the centre of the plant but as the season advances all the areoles down to about a third of the height of the plant will probably produce buds in many cases. The botanical name is derived from two Greek words meaning naked calyx and refers to the absence of spines on the perianth tube.
Gymnocalycium damsii as sold by nurserymen is a very variable specimen having characteristics belonging broadly to both the true species and G. mihanovichii. and it is certain that extensive hybridization has occurred between these two species making it difficult for collectors to be sure of obtaining the type unless they buy from a specialist. The plants sold by nurserymen as G. Jamsii have low globular plant bodies which produce small offsets from the areoles. They seldom attain any great height in cultivation and are mid-green in colour with ten broad rounded ribs. These ribs have a slight chin below each areole and they do not normally carry the diagonal bands which are associated with G. mihanovichii. The flowers are extremely variable, ranging from pink through white to green. Those with green flowers are more closely related to G. mihanovichii. those with white flowers are more true to type, and those with pinkish flowers have yet another species mixed in with them.
Gymnocalycium denudatum not only has a naked calyx but also a great paucity of spines on the stems. It is referred to occasionally as the spider cactus because of the somewhat spidery appearance of the few spines it has got. The plants have about five ribs, occasionally more in cultivated specimens. It makes a taller plant than the preceding species, ultimately attaining a height of 4 in (10 cm) even when cultivated. The bodies are a dark green and the ribs are broad and much rounded at the edges. The weak spines are yellowish in colour and there is usually no central spine at all. The flowers, which are green on the outside and pale green within, are produced fairly freely. Because of its lack of spines and consequent difficulty in protecting its surface against direct sunlight this species will benefit from a little shade during the holiest part of the spring and summer.
Gymnocalycium gihbosum is extremely typical of the group called chin cacti. The plant bodies, which are normally solitary, arc at first globular bul become cylindrical with age. a feature which is quite distinctive. The general colour of the stems is bluish green and Ihe ribs, which number between twelve and fourteen, are straight and prominently notched into chin-like tubercles which give this group of plants its common name. The flowers are white, often suffused with pink, but the plant has to be well established and up to 3 in (8 cm) in diameter or so before it can be relied upon to produce
As with G. damsii. there is a great deal of variation in this species and this extends both to the spine formation and to the colour of the flowers. The climate in which it grows in Southern Argentina is cool and moist and like the preceding species it does not enjoy very hot summer temperatures; because of its normal environment it is suited to growing in Britain bul is nevertheless a very slow-growing species compared with other members of Ihis genus.
Gymnocalycium andreae is another plant with bluish-green plant bodies but it branches readily and the bodies are permanently globular, not elongating with age. The flowers are a pale yellow colour inside, a colour which is also found in the rare
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