May 31, 1913 



HOETICULTUKE 



823 



LIGHT IN GREENHOUSES 



The evolution of the modern green- 

 house from the older types has re- 

 sulted in a marked change in green- 

 house management which has brought 

 about the use of larger glass and 

 larger houses, and a relative decrease 

 in the size of the framework, or any- 

 thing tending to produce shade, made 

 possible by the introduction of iron. 

 As a consequence of this improvement 

 in greenhouse construction crops are 

 grown better and more cheaply, for in 

 these large, well-lighted houses they 

 are less exposed to sudden changes 

 and are consequently less susceptible 

 to disease. 



There is no factor associated with 

 the construction and management of 

 the greenhouse which does not pos- 

 sess a bearing on crop production. 

 Heat, light and moisture are especially 

 important, and the size and direction 

 of the house, amount of air space, sys- 

 tem of ventilation, size, quality and 

 angle of the glass, purloins, posts, etc., 

 are also important, as are air and soil 

 moisture and the physical, chemical 

 and biological features of the soil. 



Practically 95 per cent of the vari- 

 ous substances which make up the 

 plant are derived from the atmosphere 

 through the chemical action of light 

 on the green coloring matter or chlo- 

 rophyll bodies, primarily located in the 

 leaves. This process is termed carbon 

 assimilation or photosynthesis, and 

 consists in the taking in of carbonic 

 acid and the exhalation of oxygen. 

 Artificial light affects plants in propor- 

 tion to its intensity and nature of its 

 rays, and since it differs from sunlight 

 very materially, it cannot be substi- 

 tuted for sunlight to good advantage 

 unless some of the more objectionable 

 rays are screened out. 



Plants make the most growth in the 

 night or in darkness, and the least in 

 the daytime. Although light has a 

 marked inhibitory effect on growth, 

 it favors the development of mechani- 

 cal or supportive tissue, which is able 

 to resist disease. Lack of light in- 

 duces the formation of weak tissue by 

 restricting its development, and many 

 plants are rendered more immune to 

 attacks from saprophytic and para- 

 sitic fungi by exposure to proper light 

 conditions. 



For some years we have been mak- 

 ing investigations on the relative 

 value of morning and afternoon light, 

 together with a study of the light in 

 greenhouses and out-of-doors and its 

 relation to plants and their diseases. 

 In these experiments we have used 

 many methof's for determining light 

 intensity which it is not necessary to 

 discuss here. There are several meth- 

 ods which may be employed to deter- 

 mine the influence of light on plant 

 growth and the relative value of morn- 

 ing and afternoon light. The differ- 

 ences in light intensity are not uni- 

 form from day to day. from month to 

 month or from year to year, the dif- 

 ference being greater some months 

 than others. Daily observations in the 

 greenhouse covering a whole year at 

 corresponding periods, viz.. from 9 A. 

 M. to 12 M.. and from 12 M. to 3 P. M., 

 showed a difference of 10 per cent in 



favor of the morning light, and those 

 made in the open air from sunrise to 

 noon and from noon to sunset showed 

 practically the same difference. The 

 greatest difference was shown in the 

 months of November and December, 

 the former giving a difference of 20 

 per cent and December a difference of 

 27 per cent. These records were ob- 

 tained by the chemical method; i. e., 

 by exposing solutions sensitive to light 

 and carefully determining by titration 

 the amount of decomposition. The re- 

 sults obtained by tabulating the yield 

 of cucumbers in houses running north 

 and south, where the crops on the 

 east side shaded those on the west in 

 the morning, and those on the west 

 shaded those on the east in the after- 

 noon—showed a difference of 15 to 30 

 per cent in favor of the east side 

 plants, due to the fact that those ob- 

 tained more light. These percentages 

 are not too high for short periods, 

 since very often the light is 30 per 

 cent more intense in the morning than 

 in the afternoon. Data obtained from 

 the measurements of the radii of tree 

 stumps, some of which were 200 years 

 old, showed a dilTerence of 17 per 

 cent, which is undoubtedly the most 

 reliable average obtainable. 



Investigations relating to the amount 

 of light excluded by glass and shadow- 

 casting material showed that this may 

 vary from 13 per cent to 36 per cent 

 or more, depending on the quality and 

 condition of the glass, and other fac- 

 tors. It was found that second-quality 

 glass transmits 18 per cent less light 

 than first quality, that third quality 

 transmits 33 per cent less than first 

 quality and 15 per cent less than sec- 

 ond quality. 



The deterioration of the glass per 

 annum is probably considerably less 

 than 1 per cent. In one test, where 

 a comparison was made of a house 

 one year old and of one thirty years 

 old, there was a difference of 8 per 

 cent in the light transmitting proper- 

 ties, and in another instance there 

 was a difference of 13 per cent be- 

 tween new and old glass which was 

 dirty and had been in use many years. 



The light transmitting properties of 

 high roofs were found to be greater 

 than those of flat roofs, one observa- 

 tion made February 8th showing a dif- 

 ference of 18 per cent in the light 

 transmitting properties of roofs with 

 angles of 32 degrees and 45 degrees, 

 respectively. 



Of the various reflecting surfaces ex- 

 perimented with, aluminum bronze was 

 found to be the best, followed by 

 white paint and dull black. Aluminum 

 bronze reflected about 12 per cent 

 more light than white paint. 



Our experiments showed that the 

 sky is an important factor in the re- 

 flection of light. Tests made to com- 

 pare the light from the sky with dif- 

 fused light, etc., showed considerable 

 difference in favor of the light from 

 the sky. The experiments also proved 

 that considerable light is reflected 

 from snow, for, where the reflected 

 light from snow was excluded from 

 our recording apparatus, the light in- 

 tensity was materially decreased. 



The loss of light from lapping glass 

 was found to be about 11 per cent. 



Numerous tests of the light inten- 

 sity at different distances from the 

 roof show that there is hardly any dif- 

 ference between five or thirty feet from 

 the roof, although just under the glass, 

 owing to irregularities of the surface, 

 the intensity is modified. Tests of a 

 large number of houses here and 

 there have shown that as a rule the 

 older the house, the less light it trans- 

 mits. The best light is obtained in 

 houses with large glass and light 

 frames containing the least possible 

 amount of shadow-casting material. 

 The curvilinear type of construction 

 gives the best results in this respect. 



A house glazed with 24 x 24, 20 x 30, 

 or 16 X 24-inch glass, placed length- 

 wise, obtains better light than one 

 having sashbars close together or 

 smaller glass. 



Double glazing, which is still found 

 in some sections, is impractical from 

 all points of view, dust and moisture 

 being collected between the two lay- 

 ers, often making the light very poor. 

 In the old, primitive sashbar houses 

 in which small, inferior glass was 

 used, so much light was shut out that 

 it required much longer to mature 

 crops than it does nowadays. The ten- 

 dency in modern houses to use better 

 quality, larger glass, and less shadow- 

 casting material, has proved most val- 

 uable. In some sections third quality 

 glass is used, but we do not consider 

 it a wise investment, especially in the 

 North, where the light is inferior to 

 that in the South. 



Since morning light is better than 

 afternoon, greenhouses running East 

 and West should be placed from 15 to 

 30 degrees North of East to obtain the 

 superior morning light. Less risk is 

 run also in syringing the plants in 

 these houses. We believe that East- 

 and-West houses are better adapted 

 for growing certain crops than North- 

 and-South houses. The methods of 

 trellising certain crops could also be 

 changed to get better light exposure. 

 A few days of cloudy weather often 

 hold a crop back and prevent its ma- 

 turing when the market is at its best. 

 Even under good weather conditions 

 many of our crops do not obtain sufiB- 

 cient light during the season of short 

 days, and anything of a practical na- 

 ture to better the light is worthy of 

 attention. 



Many greenhouse operators do not 

 realize the full significance of the ef- 

 fect of light on the development of 

 crops. There is a close relationship 

 between light intensity and carbon as- 

 similation in foliage and also between 

 carbon assimilation and growth. The 

 optimum light conditions for carbon 

 assimilation vary in different crops. 

 Palms, ferns, etc., require less light 

 than cucumbers, and in some cases 

 outdoor crops probably receive more 

 light in summer than they require, but 

 most of our economic plants grown 

 undpr glass do not receive sulflcient 

 light during the shorter days of the 

 year to make maximum growth. 



George E. Stone. 

 Amherst, Mass. 



