LIGHT 



LIGHT 



1857 



movements of leaves and flowers (begonia and sun- 

 flower) which adjust themselves to certain angles as 

 regards the source of light. 



Phototaxis and photonastie. 



The reacting or orientating response of motile (bac- 

 teria, swarm spores, and the like) or free-moving organs 

 (chloroplasts) to a unilateral light stimulation is called 

 phototaxis. This is a common mode of light reaction 

 in plants. Plant movements which are not dependent 

 upon the direction of light stimulus but are due to 

 changes in the intensity are called photonastie move- 

 ments. Under this category may be placed the move- 

 ments of stomata which open under illumination and 

 close in the darkness, also certain movements of 

 etiolated seedlings when subjected to light. 



The pathological effects of light. 



The pathological effects of light on plants are a much 

 more important factor than is generally realized. Many 

 pathological conditions of plants are brought about by 

 lack of light, and in some cases excess of light may pro- 

 duce injurious effects. This is shown in the case of sun- 

 scald, which occurs on various trees. Some plants are 

 so perfectly adapted to forest conditions that they can- 

 not endure direct sunlight without injury. When they 

 are exposed to direct light such as is caused by forest 

 thinning, they are likely to sun-scald badly, but more 

 often difficulties arise from lack of light. Tissue devel- 

 oped under poor light conditions is more likely to be 

 affected with winter-killing due to non-ripening of the 

 wood, and burning from fumigation with gases is 

 induced by lack of light in poorly lighted greenhouses. 

 Lack of light during the dark months often develops 

 inferior tissue, which, when exposed to the more intense 

 light of spring, becomes susceptible to wilting. The 

 exclusion of light from part of the plants resulting from 

 crowding, often gives rise to various stem-rots such 

 as are characteristic of parsley, water-cress, lettuce, 

 and the like, and there are a large number of leaf- 

 blights and spots such as occur on cultivated plants 

 which are induced by insufficient light. Lack of light 

 induces the formation of various mildews on plants and 

 is conducive to damping-off in many cases. It often 

 causes disease of plants growing in dry soil as a result 

 of excessive transpiration; in short, every greenhouse 

 grower must regulate the growth of his crops according 

 to light conditions in order to eliminate the possibility 

 of disease. 



Light as a factor in greenhouse construction and manage- 

 ment. 



In the growing of plants under glass, which con- 

 stitutes a large and constantly increasing industry, the 

 problem of light is intimately associated with the loca- 

 tion, construction of the greenhouses, management, 

 and so on. Improvements in the line of greenhouse 

 construction have been based very largely on the effects 

 of light. The early houses in the United States were 

 very crudely constructed, and in the modern, improved 

 types of houses some crops are grown in one-half the 

 time formerly employed, a fact due largely to improve- 

 ment in methods of greenhouse construction. The old 

 type of houses were chiefly sash-houses cumbered with 

 shadow-producing material and glazed with small glass 

 of inferior quality and often dirty, and widely lapped. 

 The houses were in some cases so poorly constructed 

 that they excluded from 40 to 60 per cent or more of 

 light-rays. The modern house need not exclude more 

 than 12 to 20 per cent. Some of the more or less 

 modern types of houses which have been built for a 

 number of years exclude as much as 30 per cent of light. 

 Poor light conditions alone greatly retard growth, not 

 to mention the frequent losses in poorly constructed 

 houses from pathogenic organisms which find most 

 favorable conditions for their development. 



118 



The amount of light to be found in any particular 

 location depends upon the latitude, but more particu- 

 larly upon the meteorological conditions which may 

 prevail, and the variation in this respect throughout 

 the United States is quite notable. Numerous meteoro- 

 logical observatories, without actually measuring the 

 light intensity or amount of light, have given data as to 

 the number of hours of sunshine, which is valuable in 

 comparing light conditions in various localities. These 

 records have been gathered for a considerable period 

 of time and reliable averages are at hand. The average 

 total number of hours of sunshine during the year based 

 upon data covering a long period of time is as folows: 



Hours. 



Chicago, Illinois 2,617 



Cleveland, Ohio 2,000 



Milwaukee, Wisconsin 1,865 



Boston, Massachusetts 2,493 



Nashua, New Hampshire 1,948 



Ithaca, New York 2,273 



New York, New York 2,510 



Philadelphia, Pennsylvania 2,575 



Phoenix, Arizona 3,742 



Modena, Utah 3,354 



Los Angeles, California 3,219 



This data shows great variation in the hours of sun- 

 shine which cannot be attributed to latitude alone. It 

 should be pointed out, however, that elevation con- 

 stitutes a very important factor as regards light inten- 



2147. Showing type of greenhouse constructed for obtaining 

 the maximum amount of light in the short winter days. Dotted 

 lines indicate position of sun at different periods of the day and 

 year (at noon unless otherwise indicated) . House 34 feet wide 

 and 20 feet maximum height. 



sity, the higher the elevation the better the light con- 

 ditions, and even the light intensities at an elevation 

 of 500 to 600 feet are better than those at the surround- 

 ing low country, at least during the early parts of the 

 day, but this difference gradually decreases toward 

 night. From the commercial florist's point of view, the 

 critical months in the year are November, December, 

 January and February, and even a few days of cloudy 

 weather when the crop is maturing often make much 

 difference in the financial returns. Much more variation 

 in light intensity exists in the dark winter months than 

 in other seasons of the year. This is true as regards the 

 differences existing between morning and afternoon 

 light, for during the darker months the light may average 

 30 per cent more intense during the morning than in 

 the afternoon. The percentage of possible sunshine 

 recorded during November, December, January and 

 February averages 22 per cent for Cleveland, 44 per 

 cent for Chicago, 54 per cent for New York, and 75 per 

 cent for Los Angeles. From such variations in the 

 amount of sunshine found in the various territories 

 during the critical months, it is evident that there must 

 be corresponding differences in the period of develop- 

 ment of the crops, the growth of a crop being in general 

 proportional to the amount of light it receives. 



