92 LIGHT, CONSIDERED WITH REFERENCE TO HORTICULTURE. 



roofs ; and, even about the beginning of the present century, half the roof on 

 the south side of conservatories, as for example at Southgate Lodge, was 

 frequently formed of glass, and the remaining half, on the north side, was 

 opaque as before : but this remaining half was placed at such an angle as to 

 allow the rays of the sun when highest in the firmament, and consequently 

 whenever it shone throughout the year, to reach the back wall. This, it 

 was thought by the architects of those days — Mr. Nash, for example, who 

 introduced this practice — would answer every purpose of a roof entirely of 

 glass, and at the same time would be warmer and more economical. It was 

 soon found, however, that not only the plants on the back wall, but all 

 those that were deprived of perpendicular light, did not thrive much better 

 than in opaque-roofed conservatories. 



284. From what has just been observed, the necessity of perpendicular 

 light will, we trust, be strongly impressed on the mind of our readers ; and, 

 also, the necessity, when plants in hothouses are intended to look well on 

 every side, of having every side of the hothouse of glass. A third axiom to 

 be kept in mind is, that a convex glass roof, or one with an irregular surface, 

 is, all other circumstances being alike, preferable to a roof in which the glass 

 is all in one plane. 



285. Though art has little power in increasing the quantity or mtensity 

 of light, whether direct from the sun or transfused in the atmosphere, yet it 

 possesses a considerable degree of power in increasing the efficiency on plants 

 of such light as may be transfused in the atmosphere. Thus, by spreading 

 out the branches of a tree against a wall exposed to the south, much more 

 light as well as heat is brought to act upon the leaves, than if the tree were 

 a standard in the free air ; because, in the latter case, there would be neither 

 the benefit of the reflection of the wall, nor that resulting from the circum- 

 stance of every leaf being exposed to the direct influence of the sun's rays 

 when it shone. In like manner, herbaceous plants or shrubs may be planted 

 or trained on surfaces sloping to the south; and on surfaces elevated and freely 

 exposed rather than in low and confined situations, in which light is obscured 

 by surrounding objects or by aqueous vapour. The light thrown on the 

 leaves of a plant in the open air may be increased by surrounding it on the 

 north, and part of the east and west sides, by a wall or other upright surface 

 painted white, or covered with glazed tiles or tinned iron. Practically, how- 

 ever, the grand means of increasing the efficiency of such light as there may 

 be m any given situation on plants, is by training them against walls, espa- 

 liers, or on the surface of the ground ; or, for those that cannot be conveniently 

 so trained, by removing all other plants and objects which are so near them 

 as either to obstruct the sun's rays or to interfere with circumambient radia- 

 tion. To insure the full eff^ect of the radiation of transfused light upon a 

 plant, it ought to have a free space around it, in width on every side at least 

 equal to its ovm height. No timber tree, which has not at least this space, 

 can receive from light the full influence w^hich it ought to have on its hori- 

 zontal branches; and hence (278) the trees in dense forests must necessarily 

 produce timber inferior in bulk to those of the same kinds, in the same 

 climate and soil, which are grown as single trees in parks, or in hedge-rows. 



286. In plant-structures a due proportion between light and heat ought, as 

 much as possible, to be preserved, because this is always the case in nature, 

 where both depend on the sun. It is not in our power to increase the natu- 

 ral light of the atmosphere ; for the great disadvantage to which horticulture 



