Why is there a botanic garden on the Mall?

The first botanic garden on the Mall was established in 1820 by the Columbian Institute to be a living museum of plants and promote science and learning in the city. In the 1850s, the government took over the garden, expanding it with collections from around the world. Although Congress and local officials debated moving the Botanic Garden to a larger site in the early 1900s, they ultimately decided it should remain on the Mall; as a public museum of living plants, it is a logical neighbor to the Smithsonian Institution and National Gallery of Art. 

Why is there a botanic garden on the Mall?

In 1820, Congress granted the Columbian Institute for the Promotion of Arts and Sciences five acres just west of the Capitol grounds for the creation of a botanic garden. This newspaper article describes plans for the Botanic Garden as well as the hopes for an eventual National Museum. At the time, Botanic Gardens were seen as outdoor museums where everyone could learn about useful plants. The garden may have been placed close to the Capitol because the Institute expected members of Congress to contribute plants from their home states and districts, allowing the garden to botanically represent the nation

Around 1850, the federal government took over the Columbian Institute's garden, as the Institute had disbanded and the government now had its own botanic collection to maintain. The basis of the collection were botanic specimen from the South Seas Exploring Expedition, 1838-1842. As this article explains, the plants had been housed near the Patent Office but were relocated to make room for more offices and to put the plants where they "could be seen to better advantage." The garden stayed on the Mall because it was considered a useful, educational institution.

At the same time that the botanic garden transferred from the Columbian Institute to the government, landscape architects were proposing designs for the Mall which envisioned the entire space as a botanic garden. Robert Mills created this plan in 1841; it features a series of distinct gardens, each of which was intended to display a certain kind of plant or educational ideal. Ten years later, Andrew Jackson Downing's plan was likewise made up of multiple gardens. Throughout the 1800s, botanic gardens were seen as places for education and self-improvement; these plans would have made the Mall one large educational space.

The McMillan Plan, approved in the early 1900s, called for a clear line of sight between the Capitol and the Lincoln Memorial, which meant that the Botanic Garden would have to move. The question was whether to shift its position on the Mall or move to somewhere else entirely. In addition to being a popular tourists destination, it was widely believed that George Washington had supported the establishment of a Botanic Garden in the city. The advocates for an new location, such as the area shown in this map, thought that such an important institution deserved a much larger tract of land. 

Congress quickly decided that the Botanic Garden was too important and historic an institution to move off of the National Mall. Instead, it was shifted to the south, with some talk of extending its grounds all the way to the Anacostia River. The new conservatory buildings, described in this 1932 article from the Washington Post, were state of the art, made of aluminum with electric controls for the ventilators and heat regulators. The Botanic Garden had been part of the Mall for over 100 years, and the investment in the new building indicated that the garden would remain for at least another hundred years to come.