2057. Light is admitted by constructing the roof, or cover, of transparent matter, as oiled paper, talc, or glass (the last being found much the best material), joined to as email a proportion of opaque substances, as timber or metal, as is found consistent with the strength requisite to bear the weight of the glass, resist the accidents of weather, &c. All plants require perpendicular light, but some, as many succulents and others, which throw out, or are allowed to radiate their branches, require the direct influence of light on all sides; others naturally, as creepers or climbers, or artificially, when rendered creepers or climbers by the art of training on walls or trellises, require direct light on one side only; and hence it is, that, for certain purposes of culture, hothouses answer perfectly well when the transparent covering forms only a segment of their transverse section, provided that segment meets the sun's rays at a large angle the greater portion of the growing season. This, of course, is subject to limitations and variations according to circumstances, and has given rise to a great variety in the external forms of hothouses, and the angles of their roofs. It decides, however, the necessity of placing all houses whose envelope is not entirely transparent, with their glazed side to the south.
