Invernaderos of the Real Jardín Botánico (Graells Greenhouse and Santiago Castroviejo Greenhouse)

The Real Jardín Botánico has occupied its present site along the Paseo del Prado since the reign of Charles III. Designed by Juan de Villanueva with Francesco Sabatini, it opened in 1781, and glasshouses played a central role from the start: the aim was to acclimatise tropical species brought back by the Royal Navy's expeditions. Villanueva's two invernáculos flanked the entrance to his pavilion, which also held classrooms, a herbarium and a library. The building remained devoted to scientific work until 1969 and now serves as a temporary exhibition gallery.

The Graells Greenhouse, dated 1856, is associated with the naturalist Mariano de la Paz Graells: an inscription on its cast-iron pillar records that it was built while he directed the Museo de Ciencias Naturales. It was heated by a so-called gloria system, underground channels filled with manure from the royal stables whose fermentation released warmth and humidity. Sources differ slightly on the dates of Graells's directorship, so those dates are left aside here.

The garden closed in 1974 because of its deteriorated condition, was restored in the late 1970s and early 1980s (with Antonio Fernández Alba in charge of Villanueva's pavilion) and reopened in 1981. The large exhibition greenhouse was inaugurated in 1993 by King Juan Carlos and Queen Sofía, and today bears the name of the botanist Santiago Castroviejo.

The Graells Greenhouse is a rectangular volume 23 m long, 9 m wide and 7.5 m high, with a cast-iron frame carrying the glazing. Inside, two parallel corridors are linked to each other and end in a semicircular glazed apse supported by a metal pillar. The channels of the former heating system, covered with cast-iron grilles, can still be seen in the floor. Now a cold greenhouse, it keeps a tropical climate through its southern orientation, the thickness of its north walls, its sunken floor and occasional misting.

The exhibition greenhouse, a work by Ángel Fernández Alba completed in 1993, speaks a different language: a continuous glass envelope set against a pre-existing wall and pierced by metal-framed windows. It brings together three macroclimates subdivided into ten microclimates, regulated by computerised controls and clean energy. Villanueva's pavilion, for its part, stands on granite columns and carries large glazing beneath a double-pitched tiled roof.

In the Graells Greenhouse, bananas (Musa), bird-of-paradise (Strelitzia reginae), Swiss-cheese plant (Monstera deliciosa), Chamaedorea palms and tree ferns such as Cyathea cooperi cover walls and roof around a small pond of colourful carp and aquatic plants. Mosses and other bryophytes thrive in the constant humidity.

The large exhibition greenhouse divides its collections into three zones. The desert zone gathers agaves, pitas, cacti and other succulents from the world's arid regions; the subtropical zone holds Canary Islands flora, cycads and useful plants such as coffee, guava and avocado; the tropical zone presents carnivorous plants, heliconias, palms and epiphytic orchids. Colour-coded labels show the geographic origin of each plant, and the institution has renewed the greenhouse signage.

The glasshouses stand on the north flank of the garden's upper terrace (the Plano de la Flor terrace), not far from Villanueva's pavilion. Because they maintain their own climate, they can be visited in any season. The garden, of about eight hectares, opens onto Plaza de Murillo beside the Prado Museum; allow roughly two hours for a tour of its main corners, including the Graells Greenhouse, which leads into the exhibition-tables area. The Reina Sofía museum and CaixaForum are close by, making it easy to combine the visit with a cultural outing.