Vol. XVII. No. 416. 



THE AGRICULTURAL NEWS. 



Ill 



SUNLIGHT AS A FACTOR IN EVOLUTION. 



A very suggestive paper un the inflaence of varying 

 amounts cjf sunlight as a probably potent factor in the 

 evolution of plants and animals was read by Colonel H. E. 

 Rawson, C.B., C.E., F.L.S., at a meetiog of the lloyal 

 Colonial Institute on November 20, 1917, Major Leonard 

 Darwin, R.E., being in the chair. This paper is printed 

 under the title, 'The Sun as Empire builder' in the AVar 

 Number nf Viuted Empire for .January 1918. 



As the Chairman of the meeting jiointed out at the 

 close of the reading, it is a well-known fact that certain 

 plants show variations, especially in their leaf-structures, 

 according to whether they grow in sunlight or in shade. 

 There has, however, been no reason hitherto to believe lliat 

 these variations are heritable. To gain the fullest value 

 from any series of experiments we not only want the facts 

 carefully recorded, but we want a true explanation of them. 

 Before drawing any theoretical conclusion, great caution 

 has to be exercised. It may be that under certain condi- 

 tions of sunlight, certain Mendelian factors in the germ 

 plasm are slowly killed or weakened, and thus the result 

 might be not only that a slow change would appear in the 

 flowers produced year after year, but that a change or 

 mutation might also take place in succeeding generations. 

 The chairman concluded by hoping that Colonel Itawson 

 would associate with himself some physiological botanist 

 in the enquiry into his experiments, which had evidently 

 been conducted with great zeal and perseverance. He 

 warned him, however, that he would be strictly questioned 

 on many points, as, for example, the purity of the strain 

 of the plants experimented with, and the controls adopted 

 in his experiments. 



Colonel Rawson exhibited a large number of examples 

 of what he claimed as the etlect of sunlight in changing the 

 structure, as well as the colours of flowers, foliage, and other 

 organs of plants. 



Nearly all of such variations had appeared from time 

 to time, and were known to botanists as sports, but attempts 

 to reproduce them at will, or to account for them satisfac- 

 torily, have hitherto failed. These sports had been now, 

 in the experiments described, reproduced at will over and 

 over again. The method con.sists in removing full sun at 

 selected intervals of daylight by erecting a screen at any 

 convenient distance from the plant so as to secure that full 

 sun only falls on the plant when desired, while at other 

 times it gets all the skylight which its position will allow. 

 Whenever a result is obtained, the ci>nditions should be 

 carefully noted and copied, even such trifles as the position 

 of any stakes supporting the plant. 



Colonel Rawson says that when quartered in South 

 Africa, his attention was first drawn to this subject by 

 noticing that the portion of a border to a flower bed of the 

 shrub called the Kei-apple {Ahtria Cajfra), which was in 

 shadow from an adjacent building for two hours in the 

 jnotning, died, though it was in full sunlight from 9 a.m. 

 to sunset, while the rest of the bolder, which got the full 

 benefit of the morning sun, flourished luxuriantly. The 

 same thing was observed in other gardens with the same 

 species of plant. He began therefore to make experiments 

 At Pretoria, and continued them in England to try to ascer- 

 tain whether any other plants showed a similar sensitiveness 

 to the direct rays of the sun. 



Several garden vegetables were first tried, and the 

 results showed that, if they were deprived of full sun in the 

 ■early hours of the day, sunlight during the rest of the day 

 was not able to make up the loss. The plants that got the 



morning low sun grow taster, and were stronger than those 

 deprived of it. 



These preliminary experiments led to others being 

 made to see whether the colours of flowers were affected 

 by screening the plants at selected intervals of daylight. 

 It was decided to specialize on the ordinary garden nastur- 

 tiums ( Tropacolum majui, tiibeivsiiin and minus) on account 

 of the remarkable results obtained from screening a clump of 

 twelve of these plants, which had been established for .some 

 two or three years in the garden. 



Within two years eleven out of the twelve plants had 

 Ijeen transformed into a new purple variety, which was 

 unknown at Pretoria, and the seed from them came true 

 when the same cmditions were attended to in the screen- 

 ing. Seed brought to Kngland also came true, and the 

 seeds from the twelfth plant at last also yielded to 

 screening under an English sun. This new form, now grow- 

 ing freely in Colonel Rawson's garden, has doubtless given 

 rise, he thinks, to many other forms which have appeared 

 from time to time. With the changes of colour in the 

 flower marked changes have occurred in the foliage. The 

 scent also changes with the colour. A range of colour has 

 been obtained from pure yellow to a rose-purple, and from 

 pure orange to a chocolate-purple. Varied as the colours 

 are, they can all be reduced by continuous screening of the 

 plants to one or other of the two forms, pure yellow or pure 

 orange. 



It was found that the whole plant, not only the flowers 

 and leaves, was affected by the screening, many most curious 

 variations in structure also having been obtained. Some 

 of these variations have been reproduced at will time after 

 time, and after a change of colour or structure has been 

 obtained in three successive years, a large number of seeds 

 reproduce it without special screening. For instance, a dwarf 

 purple form has been obtained which comes true from seed 

 year after year, although there was no dwarf form nor purple 

 form in these experiments when begun twelve years ago. 



Colonel Rawson considers that his experiments demon- 

 strate that with nasturtiums the yellow colour is correlated 

 with receiving the rays of the sun at a low altitude from 

 the horizon up to 35°, the red with a middle altitude from 

 3-5° to 5-5°, and the blue and purple with the sun above .5-5°, 

 and he believes that this will hold good for the colouring 

 of all flowers. 



Experiments on yellow and red tomatoes seemed to show 

 that the yield of fruit is substantially increased by attention 

 to the shading which the plants receive from tlieir surround- 

 ings at particular periods of daylight. Four times as much 

 fruit was noticed on a 30-foot length of yellow tomato plants 

 well placed for the reception of the rays of the low morning 

 and evening sun, as on a similar length parallel to it, but 

 shaded from these low rays. 



As in the case of the Kei-apple, the consideration of 

 which led to these experiments, Colonial Rawson found that 

 he was able to render nasturtium plants unhealthy or to kill 

 them altogether, or to restore the unhealthy plants to vigour 

 by selective screening during short periods of daylight. 



He affirms his belief that sunshine, shade, and darkness 

 represent the presence and absence of factors of energy as 

 important in the process of evolution as the presence and 

 absence of Mendelian units in tracing heredity. He thinks 

 that the examples of the sun's influence on plants leave no 

 room to doubt its influence upon animals and man. He 

 concludes by hoping that many others, especially in sunny 

 lands, will be induced to inike similar experiments for 

 themselves and so increase our knowledge of what lie 

 styles a 'fascinating subject.' 



