Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. rea Issued April 16, 1913. U. S. DEPARTMENT OF AGRICULTURE. BUREAU OF PLANT INDUSTRY—BULLETIN NO. 279. B. T. GALLOWAY, Chief of Bureau. THE EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH IN LOUISIANA. BY H. L. SHANTZ, Physiologist, Alkali and Drought Resistant Plant Investigations. WASHINGTON: GOVERNMENT PRINTING OFFICE, 1913, BUREAU OF PLANT INDUSTRY. Chief of Bureau, BEVERLY T. GALLOWAY. Assistant Chief of Bureau, WILLIAM A. TAYLOR. Editor, J. E. ROCKWELL. Chief Clerk, JAMES E. JONES. ALKALI AND DROUGHT RESISTANT PLANT INVESTIGATIONS. SCIENTIFIC STAFF. Thomas H. Kearney, Physiologist in Charge. H. L. Shantz, Plant Physiologist. A. C. Dillman, Assistant Plant Physiologist. 279 ; , 4 LETTER OF TRANSMITTAL. U.S. DEPARTMENT OF AGRICULTURE, BuREAU OF PLANT INDUSTRY, OFFICE OF THE CHIEF, Washington, D. C., December 4, 1912. Sir: I have the honor to transmit herewith a manuscript entitled “The Effects of Artificial Shading on Plant Growth in Louisiana,” by Dr. H. L. Shantz, Physiologist, Alkali and Drought Resistant Plant Investigations, and to recommend its publication as Bulletin No. 279 of this Bureau. This manuscript describes experiments with various crop plants which were grown under artificial shades of different degrees of density, the purpose being to determine the effect of diminished light intensity upon the growth of plants. In nearly every case a moderate reduction in the intensity of the light resulted in an increased growth as compared with plants of the same species growing in the open. It was also observed that in the case of lettuce the quality was improved by a moderate degree of shade. These results are obviously significant in relation to plant growth in the arid regions, where the light is normally much more intense than in Louisiana. Cooperative experiments of a similar nature are now in progress in Colorado, the object being to determine the importance of light as a factor in the physiology of drought resistance. The experiments here described were conducted by Dr. Shantz at the University of Louisiana while occupying the chair of botany and bacteriology at that institution. Respectfully, B. T. GaLLoway, Chief of Bureau. Hon. JAMEs WILson, Secretary of Agriculture. 279 wy, se “aS hs ae até it roe: alt ; < ce Introduction. . ? Experimental eieenee Physical conditions. - Light-...-. Temperature. . Saturation omar Summary of phy ty Serene Efiects of different ties intensities on plants. eS SS See ee Se ee eee Potato. ...- 0 Fe A = a Sake aS ara ab ae een ees, Le Ree ae OT Gori: => 7%: General condition of the plants at the end of 30 days.......-.-.-.-.---.------ General condition sree soles Tee rer mn oS ot SA te 5 | Renee cet Other Ae i Effect of shade on the Nicene GE leave es. General discussion. - Summary... Literature ee 279 LES RAT OES: PLATES. PuaTE I. Fig. 1.—Beds used in the shade experiments at Baton Rouge, La. Fic. Fig. 2.—Reproductions from photographic prints, showing the texture of the cloths used on sections 1 to 5 and indicating the relative light penetration. - II. Fig. 1—General appearance a euiien lene 30 aan eee germina- tion in bed A. Fig. 2.—General appearance of cotton plants 30 days after germination in bed B. Fig. 3.—Relative growth of mustard plants at the end of 24 days in beds A and B. III. Fig. 1.—Leaf prints of potato from all sections of beds iS a B at the end of 30 days. Fig. 2—Leaf prints of corn from beds A and B at the end of 30 days.. IV. Fig. 1.—Leaf prints of fenmee om pede A ead B sh ane endl or 30 days. Fig. 2.—Leaf prints of mustard from beds A and B at the end of 30 days. - V. Fig. 1.—Leaf sais a eon on ihedle hs saa B at “ite and ie 30 days. Fig. 2.—Leaf prints of radish from bed B at the end of 30 days... VAY: @neals appearance “of ped i iecienes 3. 2. hal re ee ne ena ae 50 Ci ySe 2k fost Scene ef TEXT FIGURES. » Diagrams of the beds.and jplamimmes: #202 -e ey eee ee . Diagrams showing temperatures at 6 a. m., averaged for the entire period of the experiments. . . Diagrams showing EEneaines ai noon, averaged aoe SNe Bree sect e of the experiments. . . Diagrams showing eee nF 6 [Oe adds averaged for the entire period of the experiments. - . Diagrams showing the een feec ae 6 a. as oe noon, and at 6 p. m., averaged for the entire period of the pecans Re . Diagrams showing the relative evaporation from a porous cup for the entire period of the experiments. - . Relative sizes of lettuce plants 50 dacs after sen con . Relative sizes of potato plants 30 days after the beginning & su. . Relative sizes of potato plants 50 days after the beginning of growth - - . Relative sizes of cotton plants 50 days after germination... -....-.--.- . Relative sizes of corn plants 50 days after germination . ......-...--- 279 6 Page. 24 24 26 26 17 18 B. P. I.—804. THE EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH IN LOUISIANA. > INTRODUCTION. In physiological studies of drought resistance light is a factor which must not be overlooked. The energy received from the sun is largely consumed in transforming water to water vapor, and hence is directly responsible for much of the transpiration from plant surfaces. The investigation described in the following pages was carried on in a humid region and wascompleted before the writer took up‘ the investigation of light requirement in relation to drought resistance in the semiarid portion of the United States. Nevertheless, since practically the same methods of experimentation have been applied in both regions and since the results obtained in Louisiana have been found to be very useful for comparison with those obtained in Colo- rado, it is deemed advisable to publish the results of the earlier inves- tigation, regarding it as complete in itself but closely related to work which is still in progress. The experiments here described were conducted at Baton Rouge, La., during April and May, 1908. The purpose of the experiments was (1) to show the effect of different degrees of shade giving a definite series of light intensities on plant growth and (2) to show to what extent the so-called shade effects were independent of the resulting changes in other physical factors such as temperature and humidity. The delay in publication * of results has to a certain extent ren- dered them of confirmatory value only, since Lubimenko (1908)? and Combes (1910) have covered part of the same ground in a very thorough way. Nevertheless, the experiments here described repre- sent a distinct contribution to the subject, since the effects of shading are not the same in different localties, owing to differences in the initial or normal intensity of the light. The results are also of interest as affording indications of the comparative light requirements of a number of species not hitherto included in experiments of this 1In cooperation with Dr. L. J. Briggs, of the Office of Biophysical Investigations of the Bureau of Plant Industry. 2 This delay was due toa desire on the part of the author torepeat the experiments before publication. Since this is now impracticable the results are published in the present form. 3 Allreferences to literature are indicated in the text by the name of the author and the year of publication. For full citations, see the list at the end of this bulletin. 279 7 8 EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH. character and in indicating the minimum amount of solar energy re- - quired by these plants. Six different plants and six different degrees of light intensity were used in the present experiments, which were so planned as to minimize the influence of the resulting changes in physical conditions other than light. EXPERIMENTAL METHODS. Two beds, A and B, 6 by 24 feet each, were prepared for seeding. On April 2, 1908, one row each of the following six plants was planted ineach bed: Radish, French Breakfast ; lettuce, New Orleans Improved Passion; potato, Triumph; cotton, Lee’s; corn, St. Charles Red Cob; mustard, Bloomdale White. As soon as the seedlings began coming through the soil each bed was covered with a framework over which cloths of different texture u 2 3 4+ 5 BED -_B. Fig. 1.—Diagrams of the beds and plantings. (Pl. I, fig. 2) were stretched so as to give five different degrees of shades, the sixth section being left uncovered so as to receive full light. Each shade was 4 feet wide and was slightly inclined, resting about 2 feet above the beds on the south side and 24 feet above them on the north side. The beds had the long dimension east and west and were closed by boards on the sides and on the ends. The diagram (fig. 1) shows the arrangement, and the general appearance of the beds is shown in Plate I, figure 1. Section 1 (east end) received the deepest shade and section 6 (west - end) was left open to the full light. Since the shades were not separated by partitions, it will be seen that a small portion of the bed between each two shades was under the next deeper shade in the morning and under the next lighter shade in the afternoon. The result was that only the center of each section received the character- istic light reduction afforded by the corresponding shade, and there 279 PLATE I. ERIMENTS AT BATON R THE LONG DIMEN .—BEDS USED IN THE SHADE Ex! Fic. 1 is EAST AND ION OF EACH BED Ss oe ; B AT THE Left. THE RIGHT WEST. TEXTURE OF = e NTS, SHOWING TH FiG. 2.—REPRODUCTIONS FROM PHOTOGRAPHIC PRI ND INDICATING THE RELATIVE LIGHT 5A THE CLOTHS USED ON SECTIONS 1 TO PENETRATION. IN PLATE II. INDICATES LEAST ILLUMINATION AND 6 NORMAL MINATION MINATION AND 6 NORMAL Zz z ee - z E = fee = fe) = z oc = = 6 i © 1e) fe) oa o = E ul! E eo Kk = <{ te 5 = 5 ep) Pe SS) 1) 260 a 2 te) QO a ae ey Zz ar S : 2 oO oy Fu = = z— Zz O 5 Sg as ee ee 5) | 6 | : LAS RS US aa A | a Bes CST Re ied ee One ees ee ee 34.0} 35.5] 32.2] 32.5 33 32 ee oe ee ee eee See 38.0; 36.0 34.5 33.5 33 32 The important facts here shown are the comparative uniformity of air temperatures in all of the different sections of bed A, and the rela- tively great variation in bed B. The temperatures of the air, of the soil at the surface, and of the soil at a depth of 8 inches at 6 p. m. show only slight differences (fig. 4). In bed A the temperatures of the soil at the surface and at the depth of 8 inches were comparatively low in the deeper shades, as a result of the wind movement and the increased evaporation caused by the fan. The temperatures of bed A show remarkable uniformity in the dif- ferent sections at all hours of observation. The only imporiant differences are to be found in the slightly lower temperatures of the 279 12 EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH. deeper shades, due to the introduction of outside air by the use of the fan. It is clear that overheating in the deeper shades was completely overcome by this method. pe es BED _A GED & pom tne =a peeps gad pene spe | pee areuiadiel Gras 5 Teak Gate Pe gs Se eee 7 Way B= 6 ie | 2 3 4 5 at 3 Fic. 4.—Diagrams showing temperatures at 6 p. m., goals for the entire period of the experiments. 20°C In bed B the differences were more marked, the most important differences having been the higher temperature of the air in the - deeper shades. SATURATION DEFICIT. The saturation deficits recorded are shown in figure 5. The differ- ences at 6 a. m. and at 6 p. m. were only slight. At noon the differ- BED A BEDALE: Fic. 5.—Diagrams showing the saturation deficit (in thousandths of an inch) at 6 a. m., at noon, and at 6 p. m., averaged for the entire period of the experiments. ences between the two beds were very marked, the saturation deficits having been comparatively low in the deeper shades of bed A and comparatively high in the deeper shades of bed B. This high satura- tion deficit in section 1 of bed B was the result of the higher air tem- 279 PHYSICAL CONDITIONS. 13 > perature. This difference in the relative evaporating power of the air in the different sections of bed B was very largely equalized in bed A as a result of the wind movement caused by the fan. Stand- ardized porous cup evaporimeters somewhat similar to those devised by Livingston (1906) showed increased evaporation as a direct result of the increase in light (fig. 6), independent of the relative evaporating power of the air. It is also interesting to note that the increased evaporation from the porous cup in the sections under the deeper shades is due in bed A to the wind movement and in bed B to the higher temperature, while the increased evaporation in section 6 in both beds was due to a third factor—increase in intensity of illumi- nation. SUMMARY OF PHYSICAL CONDITIONS. In regard to light, both beds A and B offered exactly the same range of conditions. In the two months during which the experiments were oS) = a J 2 3 4 5 6 I = 3 4 6 5 Fig. 6.—Diagrams showing the relative evaporation (in grams per day) from a porous cup for the entire period of the experiments. conducted there was approximately 70 per cent of possible sunshine at the nearest station where observations were recorded. The results show that a slight reduction in illumination is accom- panied by increased growth. In the lighter shades the occurrence of dark days would reverse the conditions between such shades and full light by reducing the light of the shades below the optimum, thus overcoming to some extent the increase in growth of plants in these shades as compared with those in full light; but in the denser shades dark days would entirely stop growth, thus increasing the differences in growth between the denser shades and full light or the lighter shades. The illumination as shown by the recorded sunshine was fairly normal or, if anything, above the normal for this locality during the period of these experiments. In bed A the use of the electric fan during the day to provide a current of air prevented entirely the changes in conditions of air tem- perature and humidity which otherwise result when shade is applied. In bed B ail of the effects upon temperature and humidity resulting from altered light conditions were operative. 279 14 EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH. During the period of experimentation the rainfall was in excess of 21 inches and the problem of water supply was never of importance. Under all shades the soil was constantly in a moist condition, the samples taken for moisture determinations showing no differences of importance to plants. The soil was well drained. In bed A the influence of the different shades was independent of their effects upon temperature and humidity, since these factors were neutralized by the use of the fan. Wind movement due to the fan was only slight, the movement having been less than 1 mile per hour in section 1 and entirely negligible in sections 3 to 6. In bed B - the results under the different shades may have been influenced by the accompanying changes in temperature and humidity. EFFECT OF DIFFERENT LIGHT INTENSITIES ON PLANTS. MUSTARD. In section 1 (n/93) the plants of mustard were unable to continue erowth after the reserve food material in the seeds was exhausted. The light was too weak to enable the plant to elaborate carbohy- drates. Growth ceased after about 25 days and the plants disap- peared at the end of 30 days. At the end of 30 days (Table IV), when the plants in section 1 (n/93) had died, no great reduction in growth was observable in the other sections with the exception of section 2 (n/15). In average height the plants (Table V) were comparatively uniform under the lighter shades at the end of 30 days. The diameter of stem (Table VI) varied considerably, being smallest in section 2 and much greater in the lighter shades. The stem diameter was not greatly increased in full light. In fact, while in bed A the stems in full light were ‘slightly thicker than those under the shades, in bed B the stems showed considerably less diameter in the full light than in sections 4 and 5. The general appearance of the plants at the end of 24 days is well illustrated in Plate II, figure 3. Growth in full light and in sections 3,4, and 5 was approximately the same. In section 2 the plants were much dwarfed,while no plants were able to survive in section 1. No appreciable difference in the plants in beds A and B could be attrib- uted to the air current of the fan and the resulting equalization of conditions of temperature and humidity under the different shades. Differences in light above one-seventh normal (n/7) seem to have had very little effect upon this plant. At the end of the experiment, 51 days from the time of planting, the mustard plants showed the best growth between sections 4 and 5, or at light intensities between n/2 and n/5. When light 279 EFFECT OF DIFFERENT LIGHT INTENSITIES ON PLANTS. 15 was reduced to n/15 or below no plants were able to survive. The mustard plants suffered severely from parasitic fungi, and on this account final weighing and measurements were omitted. LETTUCE. Lettuce was unable to elaborate any food material in section 1 and the plants died as soon as the reserve food material in the seeds had been consumed. In section 2 growth was barely possible and the plants were thin and emaciated at the end of the experiment. Let- tuce could not continue growth where the light was reduced to less than n/15. In this respect it showed somewhat less shade tolerance than the mustard. In light more intense than n/15 no great differ- ence was noted in the weight, although the plants were uniformly heavier in sections 3 to 5 than in normal light. (Table VIII.) In height plants 30 days old (Table V), as well as those 50 days old (Table Fic. 7.—Relative sizes of lettuce plants 50 days after germination. The numbers correspond to those of the shades and the letters indicate the two beds in the experiment. (Traced from photograph.) IX), were consistently taller in sections 3 and 4. The diameter of stem after 30 days (Table VI) and after 50 days (Table X) was not substantially increased in full ight as compared with light reduced as little as to n/7. The stems in n/15 light were greatly reduced in diameter. In figure 7 the relative size of the plants at the end of 50 days is shown. No consistent differences attributable .to the differences in temperature and humidity could be noted as between bed A and bed B. In the deepest shade the plants died more quickly in the bed with the fan than in the bed without, but aside from this no other differences were noted. Lettuce could not endure a shade greater than n/15. The best growth was made under the lighter shades. In full light the plants were smaller than in n/2 and n/5 light. Growth was best along the line between these two shades. In flavor only a slight change could be noted between the plants receiving full light and those receiving 279 16 EFFECTS OF ARTIFICIAL SHADING, ON PLANT GROWTH. n/2 ght, but under n/5 illumination the strong taste had almost entirely disappeared. When the light was reduced to n/7 the flavor was even better, but the plants were by no means as desirable in habit of growth as in brighter light. RADISH. In n/93 light the radish continued for 30 daysin bed B. In bed A all plants subjected to this reduction of ight had died before this date. At the end of 50 days plants were still growing in n/15 light, although the growth was very slight. The shade tolerance of radishes is about the same as that of lettuce. Growth was quite consistently better in n/2 and n/5 than in full light, the plants having been not only heavier (Table VIII) but taller (Table [X) and showing more nodes (Table XI). The plants in bed A seem to have been better than those in bed B according to measure- ments of height and weight, but observations at the end of the exper- iment failed to show any consistent difference. General observations showed that the best growth of radishes took place in sections 4 and 5, and a decided reduction in section 3 and in fulllight. There was practically no production of roots below section 3. The effect of shade could not be noted in the flavor. POTATO. Because of the large amount: of stored food material, potato plants were able to continue growth in the deepest shade for a period of 50 days. At the end of 30 days there was a noticeable difference in the height of the plants in the different shades (Table V), and by the end of 50 days the plants under shades which gave a light intensity vary- ing from n/7 to n/2 became much taller than normal, especially those in bed A (Table IX). In diameter of stem there was little difference either after 30 days or after 50 days. (Tables VI and X.) In weight there was considerable variation after 30 days (Table IV), and even more after 50 days (Table VIII). The plants gained greatly in weight during this period in light n/2 to n/7, but in light n/15 the plants weighed less at the end of 50 days than at the end of 30 days. This loss is also indicated by the shrinkage of the diameter of the stem. The number of nodes developed was greater in light n/2 to n/7, and it is evident that the increased height was due to an increase in the num- ber of nodes and not to greater length of the individualsegments. The number of nodes developed did not differ greatly in full hght and in light intensities of 1/15normal or less. (Table XI.) The height of the plants was about the same in normal light as in a light intensity of n/15. (Tables V and IX.) 279 i ee EFFECT OF DIFFERENT LIGHT INTENSITIES ON PLANTS. 17 It should be noted that the plants in bed A (that in which the fan was operated) were much taller and heavier than those in bed B. (Tables IV, V, VIII, and IX.) Since the difference showed most markedly in shade 5, which was so far removed from the fan as to make it practically impossible to detect the air current, the writer is inclined to regard it as produced by some factor other than those especially considered in this paper. The potato could elaborate no food material and in fact developed no leaves in the deepest shade. In n/15 light the leaves were very small, and no food material, or at least very little, was elaborated, since when once the plants had exhausted the reserve food supply there was no further increase in weight, the weights at the end of 50 days having been less than at the end of 30 days. With a light intensity varying from n/7 to n/2 growth was greater than in full light. Fic. 8.—Relative sizes of potato plants 30 days after the beginning of growth. The numberscorrespond to those of the shades and the letters indicate the two bedsin the experiment. (Traced from photograph.) The relative size of the plants after 30 days (fig. 8) and 50 days (fig. 9) shows the increased growths in light n/2 to n/7; also the limited growth in n/15 and n/93 light. As long as the food supply of the tuber was adequate to the needs of the plants they grew-as well under the deeper shades as in the more intense light; but as soon as the food supply failed, it became evident that a light intensity of n/15 or less wasinsufficientforfood manufacture. Small leaves, which were normally green developed in n/15 light, but apparently little or no starch formation could take place. COTTON. In some ways the results of shading cotton were more interesting than those obtained with any other plant. The weight of the cotton plants showed a considerable increase at the end of 30 days even in n/15 and n/93 light. In n/7 to normal light no great differences 279 18 EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH. were noticed. After 50 days, however, the greatest growth was recorded in n/2 light. In the full light great reduction was noticed in the weight of the plant in each case. It is especially interesting that cotton, which is supposed to be a sun-loving plant, was the only plant able to remain alive and in a healthy condition in light as weak as n/93 for as long a period as 50 days. The plants continued in n/15 light in both beds until the end of the experiment, although no appreciable increase in weight resulted. In height the best growths recorded were in light of an intensity equal ton/7 orstronger. (Tables Vand IX.) In n/93 and n/15 the plants were not noticeably taller at the end of 50 days than at the end Fic. 9.—Relative sizes of potato plants 50 days after the beginning of growth. The numbers correspond to those of the shades and the letters indicate the two beds in the experiment. (Traced from photo- graph. ) of 30 days. In each case the height in normal light at the end of 50 days was less than in n/7 to n/5 light. The relative diameter of stem is shown in Tables VI and X, and was usually greater in the weaker shades than in full light. A notice- able difference also occurred in the number of nodes developed at the end of 50 days, an increase in number having been observed in the weaker shades. (Table X1.) A good idea of the appearance of the plants at the end of 30 days in the bed with the fan is given in Plate II, figure 1, and in the bed with- out the fan in Plate II, figure 2. No appreciable reduction in growth took place until the light was reduced below n/7. The relative size of the plants at the end of the experiment (after 50 days) is shown in figure 10. 279 EFFECT OF DIFFERENT LIGHT INTENSITIES ON PLANTS. 19 At the end of the experiment no consistent effect could be noted which could be ascribed to the use of the fan. The best growth was found between n/5 and n/2 light, a marked decrease being noticeable in full light. In the shades no great reduction in growth occurred until the light was reduced to n/15, and even at the end of the experi- ment plants were alive and in good condition in n/93, although they had produced no leaves aside from the cotyledons. Cotton, therefore, showed more tolerance of shade than any of the other plants used in these experiments. CORN. Corn showed considerably more tolerance of shade during the early portion of the experiment than either mustard, lettuce, or radish, b b (b J 3 af Fig. 10.—Relative sizes of cotton plants 50 days after germination. The numbers correspond to those of the shades and the letters indicate the two beds in the experiment. (Traced from photograph.) probably as a result of the larger seed and consequent greater food supply. At the end of 30 days plants were still living in n/93 light, and in bed A plants remained alive in n/15 until the end of the experi- ment, or 50 days. On the whole, however, the best plants were pro- duced in the strongest light (Table VI1), and in this respect the results with corn differ from those with cotton, potato, radish, lettuce, and mustard. While the plants were tallest in n/7 to n/2 light, the stems had the greatest diameter in the strongest light. The most interesting fact brought out in the case of corn was that although the plants grew fairly well with as slight illumination as n/15, and although with a light intensity of n/92 plants maintained themselves in the bed with the fan for 30 days, all plants had disappeared from the portion of the beds receiving this intensity of light before the end of the experiment, and in the bed without the fan they had also disappeared in n/15 light. 279 20 EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH. At the end of the experiment it was evident that corn was the least tolerant of shade of any of the plants used, and a reduction of light to n/2 caused a decided reduction in growth. Figure 11 shows the relative size of the corn at the end of the experiment. In corn, as in the other plants, the differences due to differences in light intensity were much greater than those produced by the varia- tions in other factors. Fia. 11.—Relative sizes of corn plants 50 days after germination. The numbers correspond to those of the shades and the letters indicate the two beds in the experiment. (Traced from photograph.) GENERAL CONDITION OF THE PLANTS AT THE END OF 30 DAYS. The relative green weights based upon the average weight per plant at the end of 30 days under the different light intensities is shown in Table IV. The results for beds A and B show clearly that the growth was uniformly better in full light at this early period of the experi- ments. While the weights of some of the plants grown in the shades were above the weights of those grown in normal light, this was the exception rather than the rule. At the end of 30 days many of the plants were still growing even in the deeper shades, and at this time the green weights of the plants in most cases showed a gradual decrease from those in full light to those in n/93 light. 279 CONDITION OF PLANTS AT THE END OF 30 DAYS. 21 TasLe 1V.—Average green weight per plant at the end of 30 days, in percentages of weight in normal light. Section number and light intensity. Bed and plants. 1,n/93. | 2,n/15. | 3,n/7 4, n/5 5,n/2. | 6,n | Bed A (with fan): Oe Se a oe Re ee eR es 2 8 30 55 77 100 Dairies a eh eee 3 se 34 65 168 130 129 100 nition ene a he > tk ee | 12 23 114 60 100 LS) Sees 2 ee ae ae oe 0 50 63 7 108 100 polit hg, Re ee a ee ee eee / 0 16 69 72 7 100 Bed B (without fan): J Re ee: Se eee 0 5 32 45 94 100 Ra see os a ees ot ee we 2 33 26 33 54 57 100 0 ES ae ee Oe eee oe ee 12 31 49 123 92 100 LON OS A a ee eee 0 88 48 77 68 100 bors = oe ee eee eee 0 16 77 123 99 100 Table V shows the comparative height of the plants at the end of 30 days. The average height of the plants in the corresponding sec- tions of beds A and B shows no significant differences. Both beds show clearly an increase in height in light n/7, n/5, and n/2 and a decided decrease in n/15 and n/93 light. In general the heights recorded in n/7, n/5, and n/2 light exceed those in normal light. While the diameter of the stems (Table V1) of many plants is greater in shade than in full light, the averages show that, as a rule, the stem diameter was smaller in reduced light. In general the plants showed a more or less gradual decrease in weight and in stem diameter in passing from normal to n/93 light. In height most of the plants in light n/7 to n/2 greatly exceeded those in normal light, and also those in n/15 ton/93 light. Plants of mus- tard and lettuce died in n/93 light, radish in n/93 light in the bed with the fan, and corn in n/93 light in the bed without the fan. TABLE V.—Average height of plants at the end of 30 days, in percentages of height in normal light. Section number and light intensity. Bed and plants. | | 1, n/93. | 2,n/15. | 3, n/7. | 4,n/5. | 5, n/2. 6,n | | | Bed A (with fan): ES Ce ee es ene ae 38 48 86. 113 104 100 See eg er er. 120 149 149 160 131 | 100 Erie ee 4 See be gt ee arg 47 105 103 89 100 Maekawa 0 51 129 108 126 | 100 ye 7S ee Oa ae ep ae | 0 109 148 174 100 100 Dee ee 0 57 102 108 102 | 100 Bed B (without fan): OE EE Bae ae 0 46| — 92 110 120 100 Pen os See 102 95 95 102 102 100 pele ee. Ss 47 68 105 100 111 100 Bi a NE SE Sy SO I a 36 54 88 125 123 | 100 een on ae 0 96 169 169 130 100 Geetha eg Oh Te 0 64 115 102 102 100 22, EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH. TaBLeE VI.—Average diameter of stem at the end of 30 days, in percentages of diameter in normal light. Section number and light intensity. Bed and plants. 1, n/93 2, n/15 3, n/7. 4,n/5. 5, n/2 6,0 Bed A (with fan): OTE EE ee ae rh 17 28 59 69 86 100 OLA COM ay cers ots ate ee ere 71 82 100 82 82 100 COLORS oe oe eee ae eee 72 91 127 109 109 100 Radish (diameter of root)............. 0 U7/ 33 33 117 100 UTS FAD Ter = eet ata Sh MU Spire ir IRR ee Se 0 60 110 130 125 100 IMSTAT NS oe. aN eae eran Fe ire Ee ees 0 33 73 62 73 100 Bed B (without fan): COT aye Rane ES REDS Pies a Eee tp nm oe 0 32 59 62 90 100 1 ECOLRALEO ee Reet Sas 4 8 peek SMa 71 82 71 71 82 100 COvlone cs Ss eee ee ets ee ee a eee 73 91 145 73 100 100 Radish (diameter of root)............. 20 17 20 50 42 100 BALE DLE 2 es eA Ay pC 0; 60 110 60 115 100 IMSS Var Clee yates ener ee orca earn 0 33 91 127 127 100 GENERAL CONDITION OF THE PLANTS AT THE END OF 50 DAYS. CORN. During the later stage of the experiment corn behaved so differ- ently from the other plants that it had best be discussed separately. In Table VII are shown the average green weight, the height, and the diameter of stem at the end of 50 days. These measurements indicate a reduction in the growth of corn even in the weakest shade. With a further reduction in light, marked reduction in growth is noted until in light n/93 none of the plants survived. Corn plants were barely able to survive in light-n/15 and in bed B failed even under this illumination. | TaBLE VII.—Growth of corn at the end of 50 days, in percentages of growth in normal light. Section number and light intensity. Bed and growth. 1/93. a 2,000 los 3, n/7. 4, n/5. 5, 0/2. 6, D. Bed A (with fan): Average green weight................-. 0 35 47 84 62 100 AVerareineisittas sien Shee sae ease creer 0 87 111 87 108 100 Average diameter of stems..........-- 0 65 111 71 94 100 Bed B (without fan): Average green weight..............-.-- 0 0 7 46 94 100 Average neigh b:. 2c sant ee cacti Sco 0 0 72 96 103 100 Average diameter of stem..........--- 0 0 41 65 118 100 OTHER PLANTS. In Table VIII the green weights at the end of 50 days are shown. An increase in weight is shown in five out of eight cases in n/7 light, as compared with that in normal light. The increase is considerably ereater in n/5 light and still greater in n/2 light. A reduction of 279 i CONDITION OF PLANTS AT THE END OF 50 DAYS. 23 light to n/15 reduced the weight many times, and at n/93 only pota- toes and cotton remained alive. In general, no growth was made in light of- an intensity equal to n/15 or less after the food supply in the seeds had been exhausted. As a rule, light intensities ranging from n/7 to n/2 produced heavier plants than normal light. TaBLeE VIII.—Average green weight of plants at the end of 50 days, in percentages of \ weight of plants in normal light. | Section number and light intensity. Bed and plants. | 1, 0/93. | 2, n/15. 3, h/7. 4, n/5. 5, n/2. 6... Bed A (with fan): PORBMD See om. ents ca ae 28 44 282 139 238 100 RAR ee ee ee 0 | 18 63 91 223 100 Path hel 2) eee ee Binge ot eee 0 | 3 103 157 228 100 Leo ee Se a - A ee ee 0) 4 106 124 129 100 Bed B (without fan): LEG Ne hapa Se oS ein ee ee 23 27 160 250 146 100 UT ETS ae Pie 8 31 177 178 100 TTT DES 2 hel 2. pt i ee ae 0 1 59 119 107 100 OER ee oe eee te. Tas 0 9 147 107 | 107 100 The tallest plants (Table [X) were produced under light intensities ranging from n/7 to n/2, while those in n/93 to n/15 were generally much shorter than in full light. The ratios between height in the various shades in which an increase was observed and height in normal light were not as great as were the ratios between weight in the shades in which an increase took place and that in normal light, which indicates that the plants under the weaker shades were heavier in proportion to their height than plants grown in normal light. TaBLE 1X.—Average height of plants at the end of 50 days, in percentages of height in normal light. Section number and light intensity. Bed and plants. oe ee SS SS SS ee l l 1, n/93.. | 2, n/15. 3, n/7 | 4-4) ep yD 6,n = | | Bed A (with fan): ' tated Rt Petes 2 os ee 112 100 179 171 175 100 Paceesdame hee eal gs. NO ae 0 66 108 130 133 100 ESE a SORES 2 6a Bien a 0 52 165 178 126 100 eipe Peo en Get ee 0 91 145 136 97 100 Bed B (without fan): ities ts! Be. Se re 108 88 142 165 125 100 ER EV ie. tee ne ee ee ee 32 38 122 108 130 100 cet Soe ea ee 0 26 96 117 117 100 The greatest diameter of the stem (Table X) was found in plants grown in the weaker shades, with one exception. With a reduction to n/5 or less, most of the plants showed a decided reduction in stem diameter. 279 24 EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH. TaBLE X.—Diameter of stem at end of 50 days, in percentages of diameter of stem in normal light. Section number and light intensity. Bed and plants. 1, n/93. 2, n/15. 3, n/7. 4, n/5. 5, n/2. 6, n. Bed A (with fan): POlAtO are Cee eee ee EE 53 67 93 80 107 100 Cotbomse eset a. SER ies ne ees 0 86 143 171 143 100 UB U GLICO ree ee Ss ys ee 0 32 84 105 95 100 Bed B (without fan): Otatoe 5 Saas eee Bet deere 80 67 86 93 80 100 COttOnet Ace ee eee ae acne 71 71 86 86 143 100 Methuen Mees ec eet eect aes Soom 0 32 84 84 105 100 In number of nodes (Table XI) the plants grown in n/2 to n/7 light usually exceed those grown in normal light except for cotton in n/7 light. With a light reduction of n/15 or more the number of nodes produced was greatly reduced except in the case of pore TaBLE XI.—Average number of nodes per stem at the end of 50 days, in see of number elon in normal light. Section number and light intensity. Bed and plants. 1, n/93. 2omyil5: BEIT 4, 0/5. 5; n/2. 6, n. Bed A (with fan): LOCATOR se see eee, eee 84 103 168 161 136 100 potter. Seta eee Pal ey 9 tao an SL jw i A Seta 55 91 127 146 100 Betis eel Py eh Uk MUR MEU Nae PR eee 0 67 107 133 120 100 Bed e er fan): (POCA ne Be Nee et On eee ak ee 97 91 116 148 123 100 Cottons = he ae ie Se Saree 18 18 91 109 127 100 IRGYOII NRE Saat SSeS mE eS Emee Sea aor Dae 0 40 107 133 107 100 EFFECT OF SHADE ON THE THICKNESS OF LEAVES. Leaf prints made by exposing a series of selected leaves for a suffi- cient length of time to produce a print on a photographic “‘printing- out”’ paper, give a very good idea of the relative absorption of light by the different leaves. Leaves which have developed in full light are thicker and give lighter colored or whiter prints than the thinner leaves which have been grown under shades. In corn (PI. III, fig. 2), though a slight difference can be noted as between normal (No. 6) and n/2 (No. 5) light, and a more marked difference as between n/2 and n/5 (No. 4), practically no difference can be noted in the shades of the prints from leaves produced under a light intensity of less than n/5. Only a comparatively slight differ- ence is shown in this series of prints, which indicates that in corn there was only a slight modification in thickness of the leaf and density of the chlorophyll as a result of this variation in the physical environ- ment. 279 Bul. 279, Bureau of Plant Industry, U. S. Dept. of Agriculture. PLATE III. Fic. 1.—LEAF PRINTS OF POTATO FROM ALL SECTIONS OF BEDS A AND B AT THE END OF 30 Days. SHADES AS NUMBERED; 1 INDICATES LEAST ILLUMINATION AND 6 NORMAL LIGHT. Fig. 2.—LEAF PRINTS OF CORN FROM BEDS A AND B AT THE END OF 30 Days. SHADES AS NUMBERED, RANGING FROM 2 (N/15 LIGHT) To 6 (NORMAL). Bul. 279, Bureau of Plant Industry, U. S. Dept. of Agriculture. PLATE IV. Fic. 1.—LEAF PRINTS OF LETTUCE FROM BEDS A AND B AT THE END OF 30 Days. SHADES AS NUMBERED, RANGING FROM 2 (N/15 LIGHT) TO 6 (NORMAL). Fia. 2.—LEAF PRINTS OF MUSTARD FROM BEDS A AND B AT THE END OF 30 DAYS. SHADES AS NUMBERED, RANGING FROM 2 (N/15 LIGHT) TO 6 (NORMAL). EFFECT OF SHADE ON THE THICKNESS OF LEAVES. 25 The potato leaf prints present a very uniform gradation, being lightest in the case of leaves produced under full light and darkest in those produced under the deepest shade. (PI. III, fig.1.) Inthe older leaves a marked difference was noted between those produced under light intensities of n/2 and n/5. The prints from leaves under n/2 were almost as white as those grown under full light, while those in n/5 were almost as dark as from the leaves grown in n/7 or below, which shows a comparatively great morphological variation as a result of changed conditions. In cotton (Pl. V, fig. 1) there was a rather uniform gradation in darkness of the print, passing from full light to the deeper shades. The most marked difference occurred between normal (section 6) and n/2 (section 5) light. In the cotyledons this difference is not so marked as in the later leaves, but there is a great although gradual variation from -n/93 to normal illumination. Cotton showed consid- erable morphological differences in both the cotyledons and the foliage leaves. Mustard shows a very gradual although slight difference in the leaves in passing from n/93 to normal light. (PI. IV, fig. 2.) This indicates but little ability to modify the leaf morphology to meet changed conditions. Radish shows a marked difference in the leaves produced under normal hght as compared with those developed under n/2 and corre- sponding differences under each deeper shade. (Pl. V, fig. 2.) The same is true of lettuce. (Pl. IV, fig.1.) In each case profound mor- phological changes resulted from altered environmental conditions. It is especially interesting to note that the leaf prints for a number of these plants indicate marked morphological differences such as have been noted in comparing sun and shade leaves of the same spe- cies (Stahl, 1883). Two of the plants used, corn and mustard, show little variation, and these plants are the ones which have proved the least productive in shade. All of the others—cotton, potato, radish, and lettuce—show pronounced variations or morphological adapta- tions, and these are the plants whick have shown increased produc- tion when grown in shade. GENERAL DISCUSSION. Lubimenko (1908) showed clearly that in France a slight reduction of illumination produced an increase in the production of dry matter in the great majority of the plants tested. Shades giving only a shght reduction of light intensity were employed, and no attempt was made to equalize other conditions. The work of Combes (1910) is far more exhaustive and brings out the fact that some of the plants 279 26 EFFECTS OF ARTIFICIAL SHADING ON PLANT GROWTH. were immediately retarded in growth by the application of shade. This was not true for the ordinary mesophytic plants but for such plants as Salsola tragus, Amaranthus blitoides, and Atriplex canescens, which are all xerophytic, sun-loving plants. Most of the species used showed increased growth in shade. Combes (1910) concludes from his experiments that seedlings are more tolerant of shade than are adult plants. From the data obtained both by Combes and by the writer it appears likely that. tolerance of shade is largely determined by the ability of the plant to manufacture food. If this be the case, seedlings, because of the food stored in the seed, would be expected to show much greater tolerance than adult plants, which are entirely dependent upon photosynthésis to support further growth. The results of the experiments described in this paper make it seem probable that only during the periods of most intense illumina- tion was a light intensity of n/15 sufficient to permit the elaboration of starches in any of the plants grown. (Pl. VI.) Moreover, none of the native plants which grew as weeds at the edge of the beds under the different shades were able to manufacture starch when the light intensity was below n/15. It was also evident that where the illumination exceeded n/15 cotton, lettuce, potato, and radish were able to elaborate organic matter as readily as under full light. Careful observation of the growth of these plants for the period during which the.experiment was continued showed in practically all cases an increased growth where there was a slight reduction of the light intensity. A decided increase in growth could be noted in passing from normal to one-half normal light in all cases with the exception of corn. In general no reduction in growth occurred when light was reduced to n/5, and usually none took place at n/7. But when the ight was ~ reduced to n/15 a very marked falling off in growth oceurred (Pl. VI) and in most species the plants disappeared entirely under the shade, giving an illumination of n/93. Mustard made its best growth in light of an intensity between n/5 and n/2, and radish between n/7 and n/5, with a decided reduction of growth in full light. Lettuce grew best where the light was between n/2 and n/7, the plants under full ight being smaller by comparison. Cotton produced the largest and most vigorous plants between the shades corresponding to n/5 and n/2 light intensity, the plants in full ight having been much smaller. In n/15 growth was almost checked, while in n/93 the plants continued alive but were unable to elaborate any food material, Full light seemed unfavorable to maximum growth in the potato but not in corn. While the potato and corn were able to maintain 279 Fic. 1.—LEAF PRINTS OF COTTON FROM BEDS A AND BAT THE END OF 30 DAys. SHADES AS NUMBERED, RANGING FROM 2 (N/15 LIGHT) TO 6 (NORMAL), Fic. 2.—LEAF PRINTS OF RADISH FROM BED B AT THE END oF 30 Days PRINTS OF BED A EXHIBIT NO ESSENTIAL DIFFERENCES FROM THOSE SHOWN. SHADES AS NUMBERED, RANGING FROM 2 (N’/15 LIGHT) TO 6 (NORMA PLATE VI. Bul. 279, Bureau of Plant Industry, U. S. Dept. of Agriculture. ‘SAV OG JO GNQ SHL Lv ‘(SLHOIT S6/N ANV ‘GL/N ‘Z/N) | aNv ‘g ‘€ SNOILOSS ‘W Gag JO SONVYVaddy IVYaNa5 GENERAL DISCUSSION. 27 themselves in the deepest shade in the early part of the experiments, they made practically no growth in a light intensity of n/15 or less. The effect of the reduction of light upon the flavor of lettuce was very marked. With a reduction to n/5 the flavor of lettuce was noticeably improved and the growth was still somewhat better than in full ight. The flavor of radish was not perceptibly changed by shading. The effect of the equalization of temperature, due to the introduc- tion of the fan in one of the beds, was in most cases very slight. In the case of the potato the growth in the bed with the fan seemed to be considerably better than in the other. In all probability this was not due to the action of the fan itself, for the reason that in sections 4 and 5, far removed from the fan, the differences were greater than in sections 2 and 3, which were nearer to the fan. In fact, careful ob- servations of the beds at the end of the experiment led to the con- clusion that the effects resulting from changes in temperature, humidity, and even wind movement, were negligible as compared with those resulting from differences in illumination. As far as the evidence from these experiments go, the effects noted in shade ex- periments are attributable to reduction of the light intensity and not to any appreciable extent to the resulting differences in other factors. In general, we may say that the growth of the plants with which experiments were made was not as good, measured by the general appearance of the plants, height, green weight, and number of nodes, in full light as it was in the light from n/2 to n/5. Some of these plants were able to grow in a light reduced to n/7 almost as well as in fullsunlight. A reduction of the light to n/15 cut down the growth very perceptibly, although all of the plants, with the exception of corn in bed B, were able to keep alive even under this condition. When the light was reduced to n/93 the only plants which survived were cotton and potato. The potato plants were in a dying condition at the end of the experiment, and the cotton had made no growth, but was able to keep alive and in good condition even with this degree of shading. The probable reason for the failure of these plants to grow in light less than n/15 was their inability to manu- facture carbohydrates. The plants were green colored and had developed chlorophyll, but were unable to increase in size after the reserve material of the seed or the tuber was exhausted. Corn showed very little ability to continue growth even with n/15 light. This amount of light reduction apparently marked almost the limit for carbon assimilation in all of the plants used. The amount of solar energy utilized directly in the process of photosnythesis has been accurately calculated by Brown and Es- comb (1905), and is a surprisingly small proportion of the amount of 279 28 EFFECTS OF ARTIFICIAL SHADING ON PLANT. GROWTH. light which in the present experiments proved to be necessary in - order that growth could take place. They have also shown that of the incident light the greater portion is dissipated by emis- sivity from the leaves and through transpiration. A limit to the amount consumed in photosynthesis is therefore established by the limited supply of CO, in the atmosphere. Although only a fraction of 1 per cent of normal light is usually required for photosynthesis, it does not follow that photosynthesis will continue when the light intensity is reduced to this amount. The actual amount of solar energy received by the plants under the different shades in the experiments here described necessarily varied ereatly at different periods of the day. The maximum daily illum- ination would probably be the most important factor in fixing the lower limits for growth under the shades. By comparing the data given by Abbot (1911, pp. 358 and 385, fig. 72) it will be seen that the rate at which solar energy is received at Baton Rouge during the months of April and May during the brighter part of the day is ap- proximately 150 calories per square meter per second, this value including both direct sunlight and the additional aoe from the sky (Abbot, 1911, p. 307). | Since the nelbuirs shading produced by the different cloths used in these experiments was measured by means of Abbot’s pyrheliom- eter, it is possible roughly to express the results in terms of total solar energy received by the plants under different shades. In these experiments growth was best when the solar energy during the brighter part of the day was so reduced as to range from n/2 to n/7, or to range from 75 to 21 calories per square meter per second. On the other hand the lower limit of growth was somewhat below 10 calories per square meter per second (n/15). Previous measurements by Brown (1905, p. 525) show that photosynthesis was not retarded in some cases when the illumination was reduced to 7 calories per square meter per second. This accords very well with the results obtained in the experiments here recorded. SUMMARY. (1) When the illumination was so decreased as to range from n/2 to n/7 a general increase in growth resulted in potato, cotton, let- tuce, and radish, which was expressed in increased green werwht, helene, and ea: of nodes. (2) Corn made its best growth in full light. (3) When the light was reduced to n/15 or less none of the plants | tested were able to elaborate food material sufficient to produce growth after the seedling stage was passed. 279 SUMMARY. . 29 (4) From Abbot’s measurements, the total maximum rate at which solar energy was received during any considerable period while the experiment lasted was probably approximately 150 calories per square meter per second. In terms of solar energy, growth was best when the energy received varied from 21 to 75 calories per square meter per second. Photosynthesis and consequently growth practically ceased when the energy was reduced to 10 calories per square meter per second or less. (5) The apparent tolerance of shade exhibited by the younger plants was probably the result of the food supply still remaining in the seed and not of any special ability of seedlings to carry on pho- tosynthesis in weak light. (6) The effects of variations in temperature and humidity incident to shading were so slight in this experiment that they could not be detected by a comparison of the plants in the bed in which these conditions were equalized by the use of an electric fan with the plants in the bed in which no attempt was made to equalize conditions by this means. Differences in shade produced such marked effects on plant growth that the effects due to shading were entirely pre- dominant, and compared with these the resulting effects of change in humidity and temperature were practically negligible. 279 . A 4 oO ROT SES Eve ote be sey LITERATURE CITED. Apsot, C.G. The Sun. New York, 447 p., illus., 1911. Brown, H. T. The reception and utilization of energy by a ,reen leaf. Bakerian Lecture, delivered at the Royal Society, March 23, 1905. Nature, v. 71, p. 522- 526, 1905. and Escoms, F. Researches on some of the physiological processes of green leaves with special reference to the interchange of energy between the leaf and its surroundings. Proceedings, Royal Society, London, s. B, v. 76, p. 29-111, 1905. ComBeEs, R. Détermination des intensités lumineuses optima pour les végétaux aux divers stades du développement. Annales des Sciences Naturelles, Botanique, s. 9, t. 11, p. 75-252, pl. 6-10, 1910. Knrep, H., and Minper, F. Ueber den Einfluss verschiedenfarbigen Lichtes auf die Kohlensaureassimilation. Zeitschrift fiir Botanik, Jahrg. 1, p. 619-650, 1909. Livineston, B. E. The relation of desert plants to soil moisture and to evaporation. Carnegie Institution of Washington. Publication 50, 78 p., 1906. LusmeEeNKo, W. Production de la substance séche et de la chlorophylle chez les végétaux supérieurs aux différentes intensités lumineuses. Annales des Sciences Naturelles, Botanique, s. 9, t. 7, p. 321-415, 1908. Srani, E. Ueber den Einfluss des sonnigen oder schattigen Standorts auf die Aus- bildung der Laubblatter. Jenaische Zeitschrift fiir Naturwissenschaft, Bd. 16, p. 162-200, 1883. Wiesner, J. Der Lichtgenuss der Pflanzen. Leipzig, 322 p., illus., 1907. 279 ol (eee COPIES of this publication may be procured from the SUPERINTEND- ENT OF DOCUMENTS, Government Printing _Office, Washington, D. C., at 10 cents per copy