. Botany; principles and problems. Botany. THE PLANT AND ITS ENVIRONMENT 159 structures generally. The advantageous character of the ordinary phototropic responses is obvious. Not only does light affect the position of plant organs but it has a profound influence upon their structure. /The stems of green plants grown in the dark are usually slender, much elongated and provided with but little woody tissue; and their leaves are. Fig. 77.—Phototropism. A mustard seedling growing with its root in water. This plant was at first illuminated from all sides, but later from only one (shown by directio
. Botany; principles and problems. Botany. THE PLANT AND ITS ENVIRONMENT 159 structures generally. The advantageous character of the ordinary phototropic responses is obvious. Not only does light affect the position of plant organs but it has a profound influence upon their structure. /The stems of green plants grown in the dark are usually slender, much elongated and provided with but little woody tissue; and their leaves are. Fig. 77.—Phototropism. A mustard seedling growing with its root in water. This plant was at first illuminated from all sides, but later from only one (shown by direction of arrows). Note that the stem has bent toward the light and the root away from it, and that the leaves have taken up a position at right angles to the light. (After Strashurger). greatly reduced in size, long petioled and undifferentiated inter- nally. Chlorophyll fails to develop and the plant assumes a pale yellow color. This general effect of darkness is known as etiolation (Fig. 78) and begins to show itself whenever the supply of light falls below the optimum either in duration or intensity. If sufficiently pronounced, etiolation ultimately results in death. The stimulus of light upon protoplasm evidently prevents the abnormal growth which we see in etiolation, but how this effect is brought about, we do not understand. Too little illumination is thus harmful to the plant, but too much may be e(iually so through its effect upon proto- plasm. To the blue, violet, and ultra-violet raj^s living substance is particularly sensitive, and in many plants the position or. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Sinnott, Edmund Ware, 1888-. New York, McGraw-Hill
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