LIGHT AND VEGETATION. i 95 



tive and tonic effect of light. Otherwise expressed, the influence of 

 ■variations of light upon plants causes adaptive reactions, and dis- 

 turbances of the nutritive processes and growth. 



In consequence of these facts the reaction of any given organ to 

 changes in the intensity of the illumination will depend upon its spe- 

 cific functions and relation to the remainder of the organism. 



The stems formed by seedlings and awakening underground 

 organs are usually surrounded by plants or other objects which cut 

 off more or less sunlight. The developing shoot can not spread its 

 leaves to the light advantageously until it has outstripped or grown 

 beyond the objects intervening between it and the light. This neces- 

 sity is one of the most important conditions in the struggle for ex- 

 istence. To meet it, a very great majority of seed-forming plants 

 have acquired the power of accelerated elongation of the stems when 

 deprived of their normal amount of light. 



Very striking examples of this reaction are offered by the awak- 

 ening corms of the Jack-in-the-pulpit (Ariswma triphyllum). The 

 conns usually lie at a distance of five or six centimetres below the 

 surface of the soil, and when the growth of the large bud begins in 

 the spring the heavy sheathing scales elongate and pierce the soil, 

 opening when the surface is reached at the distance of a few centi- 

 metres. If thecorm should have been buried deeper in the substratum 

 by floods or drifts of leaves, the growth of the bud scales will con- 

 tinue until the light is reached, though it may be a distance of twenty 

 centimetres. Such growth may be seen if the corms are grown in a 

 deep layer of sphagnum moss, or in a dark room. 



After the stems emerge from the " drawn " buds they show a 

 similar attenuation, attaining a length of twice the normal. The ex- 

 cessive elongation of stems is accompanied by variations in the struc- 

 ture and contents of the tissues. The cells are generally longer, 

 while the walls are thinner. In consequence, organs grown in dark- 

 ness are very weak and easily bent or broken. Growth in darkness is 

 attended by the non-formation of chlorophyll. This is replaced by 

 etiolin, giving the plant a pale, waxy, yellow appearance. 



The adaptive elongation is not shown by all species, however. 

 It has been found that stems of beet, hop, dioscorea, and a few 

 others show no adaptations to diminished light. The adaptive 

 modification of stems elongating in darkness is developed from the 

 retarding influence exercised by light upon growth. Thus it is a 

 well-known fact that the action of certain portions of the sun's rays 

 actually impedes or checks the increase in volume known as growth, 

 though it does not influence actual division of the cells to any great 

 extent. When this retarding action is eliminated excessive elonga- 

 tion ensues. 



