. A text-book of botany for secondary schools. Botany. GERMINATION OF SEEDS 93 appearing in the word "; The term helio- tropism is often used, meaning "directed by the influence of the sun "; but while the sun is the usual source of light, it is not the only one. It should be noted that it is not light in general that acts as the stimulus, but the direction of the rays of light. The response of the stem to this stimulus is to turn directly toward the source of the light rays; that is, the stem is positively phototropic. Fig. 86 shows a bean seedling that was plac
. A text-book of botany for secondary schools. Botany. GERMINATION OF SEEDS 93 appearing in the word "; The term helio- tropism is often used, meaning "directed by the influence of the sun "; but while the sun is the usual source of light, it is not the only one. It should be noted that it is not light in general that acts as the stimulus, but the direction of the rays of light. The response of the stem to this stimulus is to turn directly toward the source of the light rays; that is, the stem is positively phototropic. Fig. 86 shows a bean seedling that was placed in a horizontal position and two hours afterward photographed. Fig. 87 shows the same plant completely inverted, allowed to grow for two days, and then photographed. In both cases the strong curvature developed in response to the stimu- lus of light is very evident, the tip of the stem in both experiments being directed toward the source of light. It should be remembered that these stimuli that influence direc- tion call forth a response only when the organ is out of line, and the response or reaction is a curve that brings it back into line. It is also important to note that the sensi- tive or irritable region of an organ is not necessarily the region in which the reaction occurs; and this means that the stimulus has been transmitted in some way from the irritable cells to those that respond, for example, by developing a curvature. Nor does the reac- tion follow the stimulation immediately; for there is an. Fig. 87.—The same seedling shown in Fig, 86, com- pletely inverted, and after two days 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 Coulter, John Merle, 1851-1928. New York, D. Appleton
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectbotany, bookyear1906
