Although some authors feel that Oreocereus celsumus and O.fossulatus are, in fact, the same species there are two quite separate forms that can be distinguished in cultivation and they are listed here for convenience as two distinct species. O. celsianus forms an upright columnar plant whose branches sprout from near the base when the plant gets older. The stems are mid- to grey-green with between nine and seventeen prominent ribs which are heavily rounded and conspicuously notched above each areole. The radial spines are completely hidden beneath the dense covering of hair surrounding the stems but the central spines, up to four in number, protrude through the hairs and can be seen quite
Oreocereus fassulatus. on the other hand, is an altogether sturdier plant with darker green stems with nine to fourteen prominent ribs and much deeper notches above the areoles than the preceding species. The spines are generally thicker and more vicious than those of O. celsianus with eleven radials and up to four centrals, which start as a sort of brownish colour and become much paler with age. The main difference lies in the distance from one another of the areoles. Those of O. celsianus are fairly close, seldom more than half an inch (1 cm) apart, while those of O. fossulaius are normally three-quarters of an inch (2 cm) apart with conspicuous white felting inside them and fewer hairs which are less matted and tangled in appearance.
There is also a species sometimes seen known as O.irollii which has fewer ribs than either of the preceding species and a rounder stem at the base.
The reason that cacti develop the very long white hairs that are characteristic of the last few species described is to reduce the amount of transpiration that takes place. Transpiration in plants is similar to perspiration in humans, but for a cactus it is essential that this process is reduced to the absolute minimum in order to conserve the maximum amount of water for the plant's own use. The long white hairs not only reflect the rays of the sun away from the stems of the plant and help to keep the plant cooler in this fashion, but whatever water is transpired from the plant is gathered on the hairs and runs down them to be reabsorbed by the soil below and eventually by the roots. In their native state cacti with these white hairs tend to grow on exposed hillsides where the sun falls directly on them, and their refreshing white appearance is best maintained in this country by giving them a position either in the home or the greenhouse where they can get as much sunlight as possible. Otherwise their re» quirements are simple and the only variety that can give any trouble in cultivation is Cephalocereus senilis, which for this reason is often grafted.
The Hylocereanae are climbing or sprawling plants whose older stems frequently develop aerial roots which support them in the wild. As a group they come from Mexico. Northern and Southern America and the West Indies and their appearance is broadly similar to one another. Their name in Greek means forest cereus and this is a reference to their preferred environment in the wild. With the exception of aporocactus and its hybrids they generally benefit from staking and it is a good idea to grow them up a mossed stake which is kept moist. A mossed stake is fairly easily made by binding pieces of sphagnum moss round a reasonably stout cane with P.V.C.-covered
Hylocerei which climb may also be grown against the wall of a greenhouse or conservatory and can even be trained, if required, like a fruit tree. Generally this planting out encourages them to grow very freely and can help to induce early flowering in many cases. It is also important to give these forest cereus a more humid condition in summer, similar to that required for rhipsalis (page 163), and a warmer position in winter. This may not be so essential when the plants are bedded out but when grown in pots it is a good idea to bring them into the house in the winter rather than to leave them with all the other cacti in the greenhouse. Pot-grown specimens will also benefit from the addition of some beech leaf-mould to the normal sandy soil that suits cereus as this gives it more texture and enables the plant to grow more freely.
: Aporocactus flagelliformis, ■
The t hanging plant and can be brought to flower even when comparatively young. It is sensible to graft this variety on to a stock such as Hylocereus undatus as the long tails have a habit of dying back to the base and this ultimately can cause the whole plant to rot off. Somewhat similar to A.flagelli-formis but far less often seen is A. conzaltii. This latter species can develop aerial roots and closer areoles [seldom more than a quarter of an inch (6 mm) apart] as opposed to A .flagelliformis which has areoles seldom less than a quarter of an inch (6 mm) apart. There is also a bigeneric hybrid between Heliocereus and A.flagelliformis sometimes sold as A.mallisonii, but probably more correctly called Heliaporus smithii, which is distinguished by its much larger flowers.
Hylocereus trigonus is a generally unremarkable plant whose main popularity lies in its use (together with H. undatus) as grafting stock as already referred to. But while the use of these grafting stocks may be of great use to the grower, since both grow profusely, their generally higher temperature and humidity requirements make them, to my mind, unsuitable stocks for most desert types of cactus. The main distinction between the two species lies in the margins of the stems, those of H. undatus being horny while those of H. trigonus are not. In the wild the sharply three-angled stems can grow up to 30 ft (9 m) in length sprawling over rocks and other bushes. The margins of the species are very wavy and the areoles, which carry about eight short spines, are borne on the crests of the undulations. Because of their sprawling habit they are not really suitable for collections with only limited space available and they also need to be some size before they will flower.
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