Subterranean plant conservatory. Rare collections to be kept under San Antonio's botanical `hats'

WITHIN the next few weeks, a group of glass and steel pyramids will begin rising from the earth just a few miles north of this city's downtown. Neither monuments to past architectural wonders nor mere flights of fancy, the ``hats'' will cover a series of underground plant rooms at the San Antonio Botanical Society's new plant conservatory. Officials of the botanical society believe that the conservatory, when completed next spring, will add a new destination to the Alamo city's visitor's map, while placing the city among the top botanical centers in the country.

``This will put us on the map,'' says Eric Tschanz, director of the botanical gardens. ``We feel we've assembled a unique collection of plants, and the conservatory will certainly help us showcase it.''

The conservatory's unique design, by internationally recognized architect Emilio Ambasz, accommodates San Antonio's intense sun in a practical manner, yet through a plan that incites curiosity and investigation.

Traditionally, plant conservatories have commonly resembled gigantic glass boxes, taking advantage of as much sun as possible. But such a design was not the best solution in semitropical San Antonio, where temperatures can top 100 degrees F. for days on end. Accordingly, Mr. Ambasz turned to one of his favorite devices -- earth-sheltered construction -- to help keep temperatures constant in both summer and winter, and to reduce the amount of sunlight beating down on the plants.

``With the amount of sun we get, this design was a perfect solution,'' says Mr. Tschanz, who came here from Ohio in 1982, shortly after plans for a conservatory were announced. ``In the North you couldn't afford to berm it all in as we have here.'' Thus the fern room, resembling a sunken pit from above, will take advantage of the surrounding earth's cooling effect in summer and warming effect in winter.

Another unusual aspect is that visitors, after entering the conservatory through one small door in an earthen wall, will approach each plant room from an outdoor courtyard featuring a large pond. After viewing a cactus room, a hot tropical room, and an orangery for raising tropical fruits, the visitor will arrive at a palm room topped by a 50-foot pyramid. Around the room's periphery, a spiral ramp reminiscent of the circular gallery at New York's Solomon R. Guggenheim Museum will lead the visitor to a frond-level view of the palms.

The San Antonio conservatory is not the first to break the glass-box mold for plant conservatories, Tschanz notes, pointing to Buckminster Fuller's Climatron -- a geodesic dome -- in St. Louis, Mo. ``But the [botanical society] wanted their conservatory to make an architectural contribution to San Antonio, and I think they've succeeded,'' he says of the $6.5 million project.

The relatively young San Antonio gardens -- plans were drawn up in the late '60s and construction began in 1975 -- were already in the forefront of a trend in public gardens toward more educational programs and native plant preservation work.

As construction on the conservatory has progressed, Tschanz and his staff have dedicated much of their time to collecting plants for exhibit in its many sunken chambers. Plants from the ``new world,'' he says, and especially Mexico and Central America, will be highlighted.

Rare and prized species among the 20,000 plants to be placed in the conservatory have come not only from other botanical gardens, but from private collectors and landowners who are pleased to make a contribution. Some landowners with rare cactuses on their property, mindful of today's international craze for the prickly plants, have required visitors seeking specimens for the gardens to wear blindfolds until the rare plants are reached.

The gardens' youthful director -- who sports constantly muddied shoes from frequent visits to the conservatory construction site -- describes a spring trip to Florida as a ``botanical buying spree.''

``We were worried we'd bought too much for the 48-foot truck we had to bring them back in,'' says Tschanz. ``But we managed to get the doors closed, and it's a good thing,'' he adds, looking toward the conservatory, which without its pyramids, from outside and down the hill, looks like little more than piles of dirt. ``We're going to need a lot of plants.''