lSo6 



LIC.HT 



Lir.HT 



than such pliuits iis thi- curuiiilirr or li-ttuw, but the 

 latter pmw iiuicli iiion> rapiilly :ui(l may jH-rhaps for 

 this ri'jisiin alono sliow ih'r ill olTccts i>f jinor lijjlil r(');anl- 

 Itss of their pholosy lit lift ic rt'iiuiroiiifnls. W'ltliiu cer- 

 tain limits, however, it ean be slated that iiholosynthe- 

 sis or eartxin-assiiiiilatioii is pro|iortionate to liglit 



2145. Showing cross section of an east and west house, about 

 36 X 200 feet, showing method of growing cucumbers. The light 

 is much superior in a house of this sort. 



intensity, :ui(l furthermore, that growth and develop- 

 ment are correlated with photosynthesis. 



The relation between i)hoto.synthesis and light 

 intensity may bo shown by vising strong contrasting 

 photographic negatives on leaves e.\po.sed to sunlight. 

 By specially treating the exposed leaves, a positive 

 ran be produced which will show that little starch was 

 formed umler the thicker portions of the negative, and 

 more umler the thinner [)ortions; in other words, the 

 formation of starch under such conditions would be 

 proportional to the amount of light received by the 

 chlorojihyll grains through the negative. 



The general effect of light on growth is to retard it, 

 though Bhuiuw, McDougal and \'ogt have been able 

 to discover a stimulation of growth under certain con- 

 ditions, while, on the other hand, lack of light or dark- 

 ness accelerates growth. The yellow rays of the apcc- 

 trum are more active in inhibiting growth than tlit' 

 violet rays, the latter having a similar effect on plants 

 to that of lack of light. Plants grow the most in the 

 night, the growth curve gra<lually rising during the 

 night and falling in the daytime. While lack of light 

 stimulates growth, plants grown entirely in the dark 

 or under poor light conditions are abnormal. Etiolated 

 plants, or those grown in the dark, are devoid of chlor- 

 ophyll, pos.sess thin stems, elongated internodes and 

 very poorly developed leaves. The mechanical or sup- 

 portive ti.ssue is little developed, and such plants 

 possess .small power of resistance. Light, on tin; other 

 hand, develops mechanical tissue and induces firmness 

 of texture. NIoreover, without light there may be no 

 incrcrase in the weight of dry matter, h<^nce seedlings 

 grown in the dark may incrca.s(; in size, but lose weight 

 from respiration or loss of carbon dioxide. 



Light is a factor in the dwarfing of alpine and arctic 

 (ilant.s and in thir ilevelopmeiit of hairs on some alga) 

 as a protection again.st too intense illumination. Some 

 plants grown in a weak light fail to [)roduc(^ fiow(^rs. 

 flawcts attracted by th(! warmth of tlie sun are mon^ 

 likely to visit flowers growing in sunshine than in the 

 deep sha^le.) Many fungi (Pilobolus) do not producl^ 

 fruiting bodies Ctporangia) until they reach the light. 

 Wiesner, who has made a thorough stuily of the light 

 refjuircments of plants, has classified some of our com- 

 mon .Sfx;cies as frdlows: 



(a) Light-ref|uiring: Alfalfa, red clover, wild carrot 

 and SO on. 



(h) Light-loving: Dandelion, plantain, lychnis and 

 DO on. 



(c) Indifferent : Blueberry, poet's narcissus, the com- 

 mon Virake, and the like. 



(d) Light-shunning: Forget-me-not, violet, anemone 

 and the hko. 



<e) Light-fearing: Wild strawberry, water violet, 

 .ind t)ir- like. 



Wiesner found that the beech, for example, reaches 

 its normal ilevelopment in one-tenth jiart of the light 

 iiiti'iisity requireil by the larch and other sun-loving 

 plants. 



Light r<>quirements are alTec^ted by a decreiuic in 

 teiniicrature: e.g., the maple in Norway re(iuires ten 

 times as much light as in Vienna for its norniai develop- 

 ment, and in general one may say, the farther north or 

 the higher above sea-level a certain plant grows the 

 greater l)e(^omes its light requirement. 



The size of leaves is very much iiiHuenced by light. 

 Too intense light as well as etiolation tciKls to reduce 

 the size of thv leaves, which reach their maximum in 

 a medium light inten.sity. This is shown in the growth 

 of such croi)S its cucumbers under glass during the 

 winter. 



I'liDlolropism. 



I'hototropism or heliotropism is the term applied to 

 the response of jilants to a light stimulation from one 

 side which causes movements toward or away from 

 source of light I'hototropic movements are of much 

 biological significance to the organism. Most aerial 

 parts of [ilants (stems and branches) arc jjositively 

 phototropic, i.e., they bend toward the source of 

 light, while roots are usually negatively phototropic, 

 i.e., they bend away from light. For a clear' under- 

 standing of the processes of plant response to light 

 stimulation from one side, it is best to consider the 

 reaction of a simple orthotropic plant such as the coty- 

 leilon of an etiolated oat seedling. This i)lant, which is 

 remarkably sensitive, has been studied extensively by 

 Darwin, Rothert, Fitting, Blaauw anfl others. It has 

 been found in this plant that the apex of the cotyledon 

 is the most .sensitive to light stimulus, thus demon- 

 strating, as in other forms of response, the localization 

 of the jjcrceptive or sensitive zone. 



The bending toward the source of light as the result 

 of the light stimulus begins at the ai)ex, and it proceeds 

 toward the base until the cotyledon coinciiles with the 

 incident ray of light. The reaction time or latent period 

 following stimulation is one-half to one hour in most 

 cases, and the rate of transmission of the stimulus is 

 from 0.7 centimeters to 1 centimeter an hour. The mini- 

 mum time of exposure necessary to induce a reaction to 

 a light stimulus is called the "presentation time," and 

 this is depenflent on the amount of light applied, which 

 is a product of the light intensity, the exposure and 

 distanei! from the plant. For exami)le, an exposure to 

 a light of high intensity (26,520 meter candle power) 

 for one-thousandth of a second jiroduced the same 

 reaction as an exposure of forty-three hours to a light 

 of very low intensity (0,00017 meter candle power). 



2146. Showing cross section of a greenhouse used tor lettuce 

 and cucumbers. The slanting side toward the right (north) is 

 boarded and used for forcing rhubarb. The cucumber plants are 

 trained vertically. 



An organ may be positively phototropic at one period 

 of its d(!velopment, and negatively phototropic at 

 another, and it has bc^en found that an oat seedling 

 will resjKind [lositively and negatively or remain indif- 

 ferent, dip(tn<ling upon the amount of light used. Many 

 complicated forms of phototropism are observed in the 



