Exp. Sta. Rec., 10(2): 103-114 (1898)
PHYSICAL AND METEOROLOGICAL RESEARCHES, PRINCIPALLY ON SOLAR RAYS,
MADE AT THE STATION OF AGRICULTURAL CLIMATOLOGY AT THE OBSERVATORY OF JUVISY.

CAMILLE FLAMMARION
Director of the Station.

The purpose of the station of agricultural climatology is to study the sun's rays and their action upon the phenomena of plant growth. The observatory at Juvisy is constantly engaged in observing the sun and its changing surface. It measures the size of the sun spots and tries to fix the astronomical bases upon which rest the relations between the variations in terrestrial temperature and the source of all the heat and light of the earth. The station studies the absorption of heat and light rays by plants, and analyzes the action of the different colors of the solar spectrum. It observes also the different transformations of the energy of the sun on which depends so intimately the entire terrestrial life.

SOLAR RAYS AND THE DEVELOPMENT OF PLANTS.

Special attention has been given to the study of the peculiar action of the sun upon vegetation. The relative efficiency of the light rays, the heat rays, and the chemical rays has been studied. In this respect the slow rays of the extreme red of the spectrum differ widely from the rapid rays of the extreme violet. The rays which exercise the most favorable influence upon certain phenomena of vegetable life are readily investigated. For such investigations hothouses were built at the station. The glass used was subjected to a careful spectroscopic examination. Blue glass, approaching closely to violet, was obtained, which was only traversed by the rays of the extreme right of the spectrum. The red glass was almost monochromatic and was only traversed by a little orange. The green glass gave least satisfaction. The three hothouses were placed close together under the same meteorological conditions. Adjoining the houses which were covered with colored glass was an ordinary hothouse, for comparison, with full illumination. The houses were ventilated by a current of air passing from south to north through them, the object being to provide as nearly as possible natural conditions and to avoid overheating. The temperature of the air in the different hothouses was observed and the quantity of the sun's heat which traversed them was measured. The accompanying table shows the temperatures observed in the different houses on August 20, 1895:

Temperature of interior of hothouses with white and colored glass.

Time of day White Red Green Blue
  Deg. C. Deg. C. Deg. C. Deg. C.
7.30 a.m 32.0 31.0 30.7 29.5
8.30 a.m 40.0 39.5 37.0 35.0
10.30 a.m 48.0 46.0 41.5 40.0
12.30 p.m 42.0 40.0 39.0 38.0
2.30 p.m 41.0 40.5 40.3 40.2
4.30 p.m 30.0 30.0 30.0 30.0

From the above table it will be seen that the temperature of the hothouses decreased as the extreme right of the spectrum was approached. The temperature was the same in all the hothouses during cloudy days or when they did not receive the sun's rays directly. The decrease of the temperature in the blue house, as compared with the white, is explained by the fact that the absorptive power of the glass of the hothouses increased as the violet extremity of the spectrum was approached. The ability to absorb the sun's rays determines the heat in the hothouses. All rays traverse the white glass, and it is therefore in this house that the highest temperature was found. On the other band, blue glass has the greatest absorbing power, and the hothouse covered with blue glass had therefore the lowest temperature.

In 1895 sensitive plants, grown from the same lot of seed and equally vigorous, were placed in each hothouse. Their height at that time was 0.027 meter. The plant placed in the red house developed extraordinarily and attained a size fifteen times as great as that in the blue house, where the plants remained it early stationary. The red light produced the effect of a chemical fertilizer, although in this case actinic rays were absent. These plants were all equally cared for. The sensitiveness of the plant grown in the red house had attained such a degree that the slightest movement or breath was sufficient to cause the closing of the leaflets and the drooping of the pedicels. The sensitiveness diminished under the white or green color, while under the blue light the plant was almost insensitive. The plant in the red house blossomed September 24. In the white house it increased in stockiness and was very vigorous but did not increase in height. It showed flower buds, but they (lid not open. The plant under the red glass had a lighter-colored foliage than that under the white. The foliage was paler than that under the green, while the blue was much darker. The difference of the temperature between the hothouses was not very great. There were, nevertheless, several degrees difference between the white and the blue. The intensity of illumination decreased in the same proportion as the temperature rose. The height of the plants in the different houses after an experiment of 3 months was relatively as follows: Blue hothouse, 0.027 meter; white hothouse, 0.1 meter; green hothouse, 0.152 meter, and red hothouse, 0.42 meter.

A photograph taken October 22 shows at a glance the influence of different solar rays (fig. 2). The difference observed might have several causes, viz: (1) The difference in the solar rays admitted; (2) difference in temperature which, compared with that in the white house, was lowest in the blue and highest in the red house; (3) difference in the amount of light, the intensity of which was greatest in the white hothouse and least in the blue; and (4) to difference in humidity of the soil and the air, which was lowest in the white hothouse and highest in the blue.

It would be interesting to know whether the results obtained are due to these differences. If the differences depend upon temperature, they would differ according to the seasons. In the spring the temperature of the hothouses during daytime remains below the optimum for the growth of plants. The amount of growth, therefore, would follow in this order: White, red, green, and blue: and during the summer, when the temperature often surpasses the optimum for plant growth, the maximum of growth would occur in the inverse order, namely: Blue, green, red, and white. The blue hothouse, the temperature of which is nearest the optimum, should favor the greatest growth. On the contrary, the results obtained in the different houses were absolutely identical in spring and summer, notwithstanding the difference in temperature. The greatest development was always produced in the red hothouse, and the minimum in the blue, where there was almost no growth. The results indicate that the inequality in growth is not due to differences in temperature. The sensitive plant in the white hothouse owes its feeble growth to the combined action of excessive light and temperature.

FIG. 2.—Sensitive plants grown in different colored light.

In 1896 experiments were begun to compare the development of plants in white and red light under the same temperature and the same intensity of light. The same temperature was obtained in the different houses by means of screens of loosely woven white linen, which allowed only a limited passage of the sun's rays but which did not alter the quality of the rays. The subjects of the experiments were sensitive plants, maize, peas, beans, lettuce, strawberries, fig-trees, Achyrantha, Perilla, Coleus, Strobilanthes, Tradescantia, grapes, etc. Some of the most interesting results are appended. On June 13 three pots of equally vigorous sensitive plants, 0.03 meter in height, were placed in each hothouse. Their increase in height at different dates is shown in the following table:

Height of sensitive plants in different colored hothouses.

Date Red White Green Blue
Meter Meter Meter Meter
June 13 0.030  0.00  0.030  0.030 
July 22 .120  .120  .080  .035 
August 16 .380  .240  .100  .035 
August 30 .470  .270  .100  .035 
October 12 .500  .280  .100  .035 

 

FIG. 3.—Sensitive plants grown at the same temperature in different colored light.

The sensitive plant in the red hothouse attained a greater height and was more sensitive than that in the white hothouse, and its vegetative development was more advanced. It began to bloom October 1, while the plant in the white hothouse began October 12. The sensitive plant in the green hothouse remained almost at a standstill, became etiolated and, the nutrition becoming insufficient, the plant ceased to grow. The plants in the white hothouse, grown under the same conditions of temperature, moisture, and illumination as those of the red hothouse, attained a height of only 0.28 meter, while those in the red hothouse reached 0.5 meter. A photograph taken October 12, 1896 (fig. 3), shows the differences in growth under similar temperatures. It will be noticed that the sensitive plant in the white hothouse had gained in stockiness what it had lost in height. Indeed, the diameter of the stem and the surface of the leaves were larger in the plants in the white hothouse than in those in the red. The weight of the above-ground part of the plants of the different hothouses was as follows:

Effect of different colored lights on the weight of sensitive plants.

Hothouse Weight of
stems and
leaves
Weight of
an average
leaf
Diameter
of stem
  Grams Gram Mm.
White 0.400  0.600  3.0
Red 4.600  .250  2.0
Green .300  .150  1.5
Blue .150  .095  1.0

It will be seen that, notwithstanding the great height of the sensitive plant grown under red glass, its weight is about half that of the plant grown in the white hothouse.

In experiments with other plants results were obtained which differed somewhat according to the species. The results with Strobilanthes dyerianus agreed entirely with those for the sensitive plant.

Young summer lettuce was placed in different hothouses (luring June and July. The results in the white hothouse and in the open air were identical. The leaves were large, thick, of a reddish-brown color, and formed a well-rounded head. The lettuce in the red house was drawn, the leaves were long and straight, blanched, drooping, and widely separated by bug internodes. The plants in the green house increased in height a little, while the leaves were less curled than those in the red house. The lettuce in the blue house added only a few leaves, without growing at all in height. The height attained by the different plants was as follows: In the red house, 1.5 meters; in the white, 0.6 meter; in the green, 0.4 meter; in the blue, 0.1 meter. The lettuce in the red house bloomed 15 days earlier than that in the white house. Figure 4 (p. 108) shows these differences.

In the experiment with maize, young stalks measuring 0.15 meter in height were set out in the hothouses in May. They were measured July 22 with the following results: In the while hothouse, 1 meter; in the red, 0.4 meter; in the green, 0.2 meter; in the blue, 0.15 meter. These results differ from those observed in the case of the sensitive plants in that the development of maize was less in the red hothouse than in the white.

In the experiments with peas and beans the most vigorous growth occurred in the white hothouse. There was less development in the red, and the minimum of growth was obtained in the blue hothouse.

The beans bloomed and fruited in the white as well as in the red hothouse, but perished in the green and blue. Peas bloomed in all hothouses except the blue, where the plant did not grow any during the two months of the experiment. The peas in the green house remained in bloom for three weeks without fruiting. Only those in the red. and white houses fruited.

FIG. 4.—Lettuce plants grown in different colored light.

The following assortment of vines was placed in each of the hothouses: One of Chasselas, two of Frankenthal, and two of Mélinet. The Chasselas, and one vine of Frankenthal, were planted in 1894, the vines of Fran kenthal and Mélinet, in 1895. Beginning with April 10, a difference in development could be noticed, due to the unequal temperature of the hothouses. The vines in the white and red houses started earliest. The growth of stem and number of leaves of a Mélinet in the different houses (May 10) is given below:

Growth of grapevines under colored glass.

Hothouse First
branch
Second
branch
Third
branch
Number
of leaves
  Meter Meter Meter  
Red 0.80  0.83  16
Green .48  .02  8
Blue .50  .45  0.18 16
White .20  .20  12

The results with the Fraukenthal were absolutely the same. There was an extraordinary development of branches in the red house, surpassing the white in this respect. This difference was greatly increased by pinching off the branches. The plants in the green light soon stopped growing and made only a few new leaves. The plants in the blue house continued to grow slowly. To recapitulate: The vine in the white house grew but slowly; its wood was well developed; it remained short and vigorous during the entire season. The vine in the red house grew in length, but lost much in vigor; its numerous branches had frequently to be pinched back, and its foliage was but little colored. The vine in the blue house grew slowly, considering the rapid start made; it remained vigorous with large dark-green leaves. One year-old vines behaved in the same manner; the vines in the white hothouse were vigorous and developed a more luxuriant growth than those in the other houses. In general, the phenomena in case of the grapes were similar to those observed in the case of sensitive plants. However, the blue rays were not so unfavorable to the development of the vine as they were for other plants. The experiments with vines were concerned only with the vegetative growth, and not with the fruiting.

The development of Perilla was intense in the white and red houses, the plants in the white surpassing in vigor those in the red. The plants in the given house changed little during the experiments, while those in the blue remained inactive.

Experiments with Coleus showed little difference in growth. The plant in the white hothouse spread and made a magnificent ornamental plant. That in the red house, while showing less foliage, increased in height, in the green and white houses there was very little development.

The results with Achyrantha accord with those for the sensitive plant. The plant in the red house grew so much in length that its branches could hardly hold up.

Three young potted strawberry plants of the same age and vigor were placed in each hothouse in May. In June the plants fruited with the following result:

Effect of different colored light on fruiting of strawberries.

  Number
of fruits
Total
weight
of fruit
Average
weight
of fruit
Grams Gram
In the open air 70 46.40  0.66 
In white hothouse 73 46.80  .64 
In red hothouse 25 9.80  .39 
In green hothouse 12 4.20  .35 
In blue hothouse 5 1.80  .36 

Only the plants in the white hothouse and those in the open air produced any considerable number of fruits. The plants receiving the total radiation yielded five times as many fruits as those which grew under the red light. Those in the green hothouse yielded about one-tenth as much as those in the white. The weight of the fruits from the blue hothouse was insignificant. The strawberries which received only colored light were very watery and insipid. Their average weight was half that of those which had received the total radiation.

The different rays of the solar spectrum modify not only the aboveground growth of plants, but also affect the entire vegetative part of the plant. The root system of the young plants which had been grown in hothouses was poorly developed. It was greatly reduced in the red house, and in the blue there was almost no root system. The weight of the roots of the sensitive plants in the different houses, October 13, was as follows: In the white house, 5 gm.; in the red, 1.6 gm.; in the green, 0.09 gm.; in the blue, 0.05 gm.

The nutrition of the plant is to a great extent dependent upon the root system. It is partly due to this dependence that the plants in the red, blue, and green houses had so little vigor. It is well known that the red and orange portions of the spectrum favor assimilation, transpiration, and respiration of plants. The natural result of this is an increase in the vegetative activity and in the function of nutrition. It is evident, therefore, that differences of growth, vegetative activity. vigor, development, sensitiveness, and coloration of plants result from differences in radiation. Microscopic examination of sections of the sensitive plant showed that the anatomical structure and the diameter of the plants in the red and white houses differed. The epidermis was thicker, and the woody fibers of the stein were more numerous in the plant in the white hothouse, the pith was much less developed, the cells were better formed, and the partitions thicker.

Notwithstanding the great care which was taken in conducting these experiments, it was very difficult to avoid all sources of error. Glass considered monochromatic for the red as well as for blue proved unsatisfactory for the reason that some yellow rays passed through them. In further study of this subject, experiments were conducted in which the three different colors were obtained from the spectrum derived from a prism.

Plants were placed in the different regions of the solar or electric spectrum and the modifications due to different rays thus observed. For this purpose a prism was constructed capable of giving a spectrum of great dimensions, and in which could be placed such liquids as carbon bisulphid and spirits of turpentine, the refractive power of which is very high. The source of light was an electric are lamp. The use of carbon bisulphid was discontinued on account of its volatility and excessive inflammability, and as a substitute spirits of turpentine, the refracting power of which is far less, was used. The index of refraction for spirits of turpentine is 1.470496 for the red and 1.493874 for the violet. The difference in dispersion of the two is, therefore, 0.023378. A straight slit was placed against the prism filled with turpentine and a convex lens placed between the prism and the arc lamp. These were placed at a distance from the prism equal to doable that of the focus of the lens The prism was placed in the position of least deviation to the spirits of turpentine, which was in this case 30°. The spectrum reflected upon a screen 3 meters from the prism measured about 25 cm. (nearly 10 in.) in length. The arc light gave a constant intensity of 10 amperes and burned twenty hours daily. Notwithstanding the limited size of the spectrum a series of experiments was started, which permitted the verification of the results obtained in the colored hothouses. These experiments were on the growth of flax and vetches, and on the transpiration of leaves.

August 25 a small box of flax was exposed to the luminous region of the spectrum. The plants had sprouted and reached a height of 0.04 meter, and the cotyledons were slightly expanded. On August 27 the first leaves were beginning to expand in the red portion of the spectrum, and some leaves appeared in the yellow, but no change was observed at the right end of the spectrum. On September 4, ten days after the beginning of the experiments, the box with the plants was photographed and the plants were measured. In the red light the stems had attained a height of 0.085 meter, and in the blue 0.040 meter.

The temperature remained the same for all plants during the time of experiment; it varied between 10° and 30°, with an average temperature for the ten days of 18° C. On account of great difficulties encountered the experiments had to be discontinued. They were continued far enough to verify the favorable action of the red rays upon the growth of plants. This favorable action seems to be due partly to the infra-red rays. The pure green of the spectrum was very small, and on this account plants exposed to green rays received some yellow rays. The use of carbon bisulphid would have provided a more extensive spectrum.

The common vetch (Vicia communis) is very sensitive to light, and grows very rapidly under a weak illumination. Pots of three plants each were placed in different portions of the spectrum and separated by small black screens At the commencement of the experiment their height was 0.04 meter. The average height of the plaits in each pot August 15 was as follows: Red 0.09, yellow 0.0$, green 0.05, violet 0.07; and August 20, red 0.21, yellow 0.185, green 0.16, violet 0.15 meter. As in the other cases, the maximum growth took place in the red portion of the spectrum, the results obtained with the spectrum of the electric light and in the hothouses with colored glasses agreeing.

EFFECT OF WHITE AND COLORED LIGHT ON TRANSPIRATION.

In the course of the above experiments the transpiration of the leaves under different rays of the spectrum was measured. It was found that grapevines attained the maximum of transpiration in white light, there being a decrease from red to blue. An experiment on leaves of maize exposed to different regions of the luminous electric spectrum gave more definite results. The maize plants were fifteen days old; the stems were equally vigorous, and each bore five leaves. The experiment was begun September 14, with an average temperature of 20° C., and each leaf was inclosed in a glass tube. The experiment lasted 26 hours, and the transpired water was as follows:

Effect of different colors on transpiration of maize.

Color Weight
of leaf
Weight of
transpired
water
Water
transpired
per gram
of leaf
Gram Gram Grams
Red 0.135  0.208  1.540 
Yellow .102  .230  2.254 
Green with some yellow rays .095  .085  .882 
Violet .080  .024  .302 

It will be seen that the maximum transpiration occurred in the orange-yellow and the minimum in the violet portion of the spectrum.

ACTION OF DIFFERENT PORTIONS OF THE SPECTRUM ON THE COLOR OF VEGETABLE TISSUES.

The many different colors of plants are mostly due to light. The green color of the leaves, due to chlorophyll, can only be produced in the light. The blue, yellow, red, and other colors are partly due to pigments and partly to cell sap. The writer has investigated the rôle of light in the coloration of the different tissues of flowers, fruits, etc.

For these researches colored-glass hothouses and absolutely monochromatic solutions were used. The solutions were contained between the walls of special vessels in the interior of which the flowers, fruits, leaves, etc., were exposed. The red was obtained from a solution of carmin in ammonia, the green from a concentrated solution of copper chlorid, and the blue from a solution of copper sulphate and ammonia. According to Sachs, the development of the coloring matter is independent of the action of light. It is developed in the leaves at the expense of substances which are produced under the action of light. The author's experiments indicate that light acts not only oil the nutrition of the plant, but also on the coloring of the tissues. It sometimes exerts a direct action on the flower, and in this case the coloring is due principally to light.

Plants may be classified according to the cause of coloration into three groups, namely, those in which coloration is due (1) to the direct action of light, (2) to the action of the light and to the food material in the leaves, and (3) to other causes than the action of light. Chlorophyll is directly due to the action of light. It is not immediately destroyed when the plant is placed in darkness, but remains unchanged as long as the plant has not exhausted its reserve, disappearing only when the reserve material has been exhausted. Observations on flowers are to the same effect. It has been known for some time that to obtain white lilacs it is only necessary to place colored lilacs, especially the Marly variety, in a hothouse kept at a constant temperature of 15° C. Duchartre states that the lilac would become white if placed in a very light hothouse if the temperature remained constantly at 15° C. He attributes the decoloration of the flowers to the more energetic oxidation of the air of the hothouse.


ACTION OF DIFFERENT LIGHT RAYS ON COLEUS LEAVES.
A, full radiation; B, red rays; C, green rays; D, blue rays; E, open air; F, subdued light; G, diffused light; H, very dim light.

The experiments of the author were conducted on Marly and Persian lilacs, planted in colored hothouses and in the open air. The panicles were budded and at the time of planting were slightly colored. In the white hothouse the lilacs became pink and almost entirely lost their color. In the red, green, and blue houses they become absolutely white. The hothouses had a varying temperature, during the night falling to 1° and rising during the day to 25°. Lilac buds inclosed in a dark hood become discolored notwithstanding the temperature was the same as in the surrounding air. If the panicles were inclosed when already more or less colored red shades were obtained. Thus it is possible to obtain on one stem flowers of all shades between the white and violet red. If panicles already colored are placed under a colored bell jar, flowers varying from pale blue to clear red violet will be obtained. These results are neither due to temperature nor to activity of growth, but are evidently caused by differences in light.

The writer's experiments covered a great number of plants, and it was found possible to change the form, size, and color of the leaves of plants with different colored light. Among the most remarkable results were those obtained with the Coleus. In the accompanying plate some of the differences observed on this plant due to its culture, character of solar rays, or the different intensity of light are shown.

In Plate I, A represents a leaf exposed to full radiation in the white hothouse, and B one grown in the red hothouse. It can be seen that the red pigment decreased in red light, the leaf spread, and its form changed. Leaf C, grown under the green-colored glass, is diminished in size, the red pigment has disappeared, being replaced by a yellow coloration. In leaf D, grown in blue rays, the red pigment has almost completely disappeared. On the same plate are also shown four other leaves of Coleus, the first grown in the open air, the second under a slightly diffused light through a garden frame, the third in diffused light, and the fourth under a still weaker light. The transformation of the plant in this case is gradually accomplished under an attenuation of light, as in the preceding case under the influence of different rays. The largest and most curious leaf is the third. The fourth was greatly diminished and modified, having changed from poppy red with a dark edge to yellow and light green.

Comparing these eight Coleus leaves it will be seen that the leaf from the hothouse under total radiation has developed much more than the one grown in the open air; that those from the red house and diffused light show increased size and a particular coloring; and that those from the green and blue hothouses and under very feeble light have lost almost all their resemblance to the normal leaf. These experiments establish the fact that light, without the aid of any other factors, is able to modify plants.

Results not less curious have been obtained with other plants, some of which are shown in the colored plate (Pl. II). The red-flowered crassula was placed in the dark at a time when its buds were only slightly colored. It shows only a narrow, colored edge bordering a white flower. The purple leaves of Alternanthera amoena became absolutely green under the red glass. Geranium leaves lost their reddish-brown tone and changed under the red, blue, and green rays into the three following forms: In the red hothouse they were large, well cut, and pale green; in the blue, almost round and dark green; in the green, small and very pale green. Similar experiments were made with fruits by surrounding the branches with colored glasses. The same results as indicated above were produced with peaches, apples, cherries, and strawberries. In certain plants the leucites, to which their coloration is due, act according to the way in which they receive the light. Others vary under the influence of light in combination with the plant's nutrition. Still others are altogether insensible to the action of light. To the last class belong carrots, beets, radishes, potatoes, truffles, etc., the underground colorations of which are evidently independent of light.


ACTION OF DIFFERENT LIGHT RAYS ON THE COLORATION OF PLANTS.
1, Red-flowered crassula: (A) in sunlight; (B) in darkness. 2, Alternanthera amoena: (C) full radiation; (D) red rays.
3, Geranium leaves; (E) full radiation; (F) blue rays; (G) green rays; (H) red rays.

[Concluded in next number.]

Exp. Sta. Rec., 10(3): 212-213 (1898)
ACTION OF ELECTRICITY UPON PLANT GROWTH.

The question of the influence of electricity on plants has been the subject of much discussion during the last few years. The results obtained from experience were frequently contradictory. For this reason some experiments were conducted in which copper and zinc plates of 0.70 meter length and 0.45 meter width, bent at right angles, were placed at the extremities of a plate 2 meters wide and 4 meters long and stuck into the ground, the top being a little above the surface. The plates were joined by insulated copper wires. There were thus created zinc-soil-copper piles on which it is supposed that an electric current could be established. The current was rendered more active by adding a Le Clanché pile of three elements and its influence on the germination of seeds tested. August 31, 1894, 56 beans were put in rows on each plate. The current was passed through for ten hours. After this an interrupted current, sometimes during the night and sometimes during the day, was passed through the apparatus. The results are worthy the attention, as the evidence of an electric action is positive.

Effect of electricity on the germination of beans

Date Number of grains germinating
Electric band Nonelectrified
September 4  4 0
16 2
10  54 34
11  54 40
13  56 45

This experiment shows that the germination was more rapid in the electrified seed and also that a greater number had germinated. We repeated the experiments in 1895 and 1896 on peas and beans with the following results: May 11, 1895, the same number of seeds were placed in each plate. No difference could be seen during the time of germination. The peas bloomed June 16; the flowers at that time were more numerous and the plants better developed in the electrified plates than in those without. The beans bloomed July 2 over all the plates. The growth of the beans was better in the electrified portion. The peas were gathered twice and gave the following results: Average of electrified portions, 941 gm.; check, 820 gm. Part of the beans were gathered twice and yielded: Electrified, 2,900 gm.; check, 2,250 gm. For the other parts there was but one harvest, which yielded as an average for the electrified portions 1,410 gm, and check, 1,445 gm. One of the electrified portions gave a yield of 375 gm. less than the check. The results of these experiments show that the parts electrified by piles yielded a harvest from 20 to 28 per cent greater than the natural one.