MRS. MARION CRAN 'S recent talk on 'Small Gardens ' leads inevitably into her subject tonight. Speaking under the auspices of the National Gardens Gufld, Mrs. Cran will give advice to both those whose only hope of a garden is on the window-sill (exposed to the soot of tho city and itself impossibly circumscribed-until you know how to make the best of it), and to those more fortunate gardeners to whom the window-box is but a decoration, an addition, to the general gardening scheme.