I have not researched swimming pools in gardens but think they have two roles (1) in sacred gardens they are for ritual cleansing – not unlike the wuḍu which takes place outside a mosque (2) in pleasure gardens – they are for pleasure.
Christianity and the long period of medieval austerity, together with its aftermath, put a ‘dampner’ on outdoor bathing, especially in northern Europe. This had many aspects: the idea that luxury was an indulgence, the thought that the human body should be kept from view, the thought that gardens were places to grow food. These thoughts were substantially overturned in the twentieth century.

Jerry: why are you not keen on the photo of villa oplontis or is it the artwork that the girl is contemplating that you are not keen on?

[ http://www.fanpop.com/clubs/tom-and-jerry/images/5227306/title/tom-jerry-wallpaper-wallpaper ]

The Romans had pools in gardens which were likely to have been used for bathing (eg Poppea’s antics at the Villa Oplontis) but the idea fell from favour after that and I think you are correct that it returned in the 20th century. Pools were always used for bathing in Indian gardens.

Similarly with the Donnell Garden – females in swimsuits are something of a twentieth century phenomena also! Without this radical change in lifestyle and the introduction of leisure for the upper and then middle classes I am sure the pool would not have gained such prominence as an element of landscape and garden design to be used rather than contemplated.

The relationship between Corb’s model and drawing deserves an article, a PhD or a book, or all of them. I think of Corbusier as an ‘objects man’. The sketch is very much of a space but a very special kind of space: a ‘room outside’. It is composed by walls and paving, not by the other elements of landscape and garden design: landform, water and vegetation. The Donnell Garden (great to see a photo of it in use) uses four of the compositional elements (no use is made of buildings).

The image the model conveys is an obsession with the built form, while his sketch conveys quite a different relationship to the natural world. [ http://www.plataformaarquitectura.cl/wp-content/uploads/2008/08/640286582_ville-savoye.jpg ]

It would seem in California Modern the pool played much the same role as the car in revolutionizing the garden space as shown in this image of the Donnell garden. [ http://tclf.org/sites/default/files/microsites/pioneers/berkeley/images/trieb_intro.jpg ]

I am not sure if the modernist garden had the same impact in the UK?

Definitely, modernism was ‘a radical break with the past’. So was the Renaissance. It is usually dated from c1350 to c1750 but, as Wiki states, ‘The word Renaissance was first used and defined by French historian Jules Michelet (1798–1874), in his 1855 work, Histoire de France. So if historians of the 20th century are to work an an equally glacial timescale ‘we’ will have to wait until 2310 for Modernism to have a good name. Just in case anyone then researches the opinion of fuddy-duddy landscape architects in 2014, I’d like to record my view that the movement should be called Abstractionism. For me, the desire to ‘abstract’ captures a key strand in Modernism. It was a period when scientists were ‘abstracting’ the laws of nature from empirical observation and artists, inspired by their success, were trying to abstract the principles of art from the historicist sentimentality of the nineteenth century: the truths of line, shape, colour, composition, structure, time, experience and so forth.

But the attributes of modernism, the philosophy of modernism, the modern sensibility are all beyond this.

Perhaps, modern, as you first noted it describes an awareness of a radical break with the past rather than a sense of continuity with it (ie. a transition from old to new)?

I agree that ‘Modern’ is an era but do not think it is the best possible name for the era in question. In a thousand years time, is it still going to be the Modern Era. I doubt it. Names are usually allocated to art history periods in retrospect (eg Renaissance and Baroque) and I am thinking that it could be time enough for naming the work of 1910-1960.
The distinction between descriptor and designator is valid but I don’t think it solves the problem.