38．What we today call American folk artwas，indeed, art of, by, and for ordinary, everyday “folks”who, with increasing prosperity and leisure, created a marketfor art of all kinds．and especially for portraits．

(what引导的从句作主语；介词结构(art)of，by，and for…——…所有、所创和所享的(艺术))

我们今天所称的美国民间艺术确实是被普通民众所有、所创和所享的艺术。随着日渐繁荣和不断增加的空闲，这些民众为各种艺术，尤其是肖像艺术创造了一个市场。

分句1：What we today call American folkart

分句2：was，indeed, art of, by, and for ordinary, everyday “folks”

分句3：who, with increasing prosperity andleisure, created a market for art of all kinds．and especially for portraits

分句1是2的主语从句

分句2是整个长句子的主句

分句3是2的定语从句，修饰folks，中间虽然有多个插入语，但仍是一个分句

1和2嵌套

3和2嵌套