t s 



2 PLANT PHYSIOLOGY AND ECOLOGY 



88. The nature of light stimuU. The stimulatory action of 

 light is exerted primarily upon the chloroplast. To this are prob- 

 ably to be traced all of the effects just enumerated, though it is 

 impossible as yet to establish this connection in the case of the 

 turning of stems and leaves, and the periodical changes in the 

 position of the latter. In these the effect of the stimulus does 

 not appear in the chloroplast, or the cell containing it, but is trans- 

 mitted to a more remote part of the plant, where it becomes evi- 

 dent. Since such movements occur only in green plants, and 

 apparently result in placing the leaves in more favorable posi- 

 tions, it seems extremely probable that they are due in the first 

 place to the reaction of the chloroplasts to light. 



Stimuli result from a change in the intensity, direction, or 

 quality of the light. In nature, the quality of the light is very 

 little if any different in various habitats. Even in dense for- 

 ests, the diffuse light is white, not green. The direction of the 

 light is of little importance, except where the illumination is strongly 

 one-sided. This is the rule in horizontal leaves, though the posi- 

 tion really results in a difference in intensity. Plants on the 

 edges of forests and thickets are often bent toward the sunshine, 

 but within a particular habitat such movements are lacking, 

 except in the case of a few plants, as the sunflower. The change 

 in light intensity necessary to produce a response varies for differ- 

 ent species, and it is also influenced by the intensity in which 

 the species normally grows. The normal extremes of intensity 

 are full sunshine, represented b}^ 1, and a diffuseness of .002, 

 i.e., light 500 times weaker than sunlight. The food-making 

 activity of the chloroplast is so fully dependent upon the light 

 that it is affected by very slight differences of intensity. On 

 the contrary, such responses as the movement of the chloroplasts, 

 changes in leaf structure, and phototropism are produced only 

 by much greater differences. 



89. Measurement of light. Determinations of light intensity 

 are made by means of the photometer. This is a tight metal 

 box containing a central wheel upon which a strip of photographic 

 paper is fastened. The wheel is revolved past an opening 6 mm. 

 square, which is closed by means of a slide working closely between 

 two flanges. The disk or the wheel is graduated into twenty-five 

 parts, which are numbered. A line just beneath the opening 

 coincides with the successive lines on the disk, and indicates the 



