. Experimental morphology. Protoplasm; Growth. § 3] EFFECT OF EXTERNAL AGENTS UPON GROWTH 481 the quickness of response, for growth by imbibition is more rapid than that by assimilation. Also a histological study of the curving region should throw light upon this question. Among the rapid tropisms are chemotropism, especially as seen in the tentacles of Drosera and in pollen-tubes, and thig- motropism, as exhibited in tendrils. Such are probably due to diiferential imbibition. Among slow tropisms are hydro- tropism and rheotropism, b_ which are probably due largely to differential as- similati
. Experimental morphology. Protoplasm; Growth. § 3] EFFECT OF EXTERNAL AGENTS UPON GROWTH 481 the quickness of response, for growth by imbibition is more rapid than that by assimilation. Also a histological study of the curving region should throw light upon this question. Among the rapid tropisms are chemotropism, especially as seen in the tentacles of Drosera and in pollen-tubes, and thig- motropism, as exhibited in tendrils. Such are probably due to diiferential imbibition. Among slow tropisms are hydro- tropism and rheotropism, b_ which are probably due largely to differential as- similation. Traumatro- pism, geotropism, and the response to radiant en- ergy, namely, electrotro- pism, thermotropism, and phototropism, are inter- mediate in their rate, and are probably due to the combined action of assim- ilation and imbibition. Sections through the responding region show the importance of imbi- bition in certain tropisms, as, for example, geotro- pism. In such sections it is seen that the cells on the convex side are en- larged in all axes and full of watei', while those of the concave side are com- pressed so that the cells are shoved into one an- other, are diminished in size, and have a dense plasma (Fig. 140). A typ- ical set of measarements of the dimension of the cells in the curving region, compared with normal cells, is given by Ciesielski ('72) as follows : — 2i. Fig. 140. — A section of a tropic radicle taken in tlie plane of curvature, at the region sg, Fig. 106. ep, epidermis ; rp, parenchyma ; gbs, sheath of the iibro-vascular bundle; Izb, flbro- vascular bundle; h, wood-cells; </, vessels. Those cells which lie next the nadir (o) are* smaller than those turned toward the zenith (6). The latter appear stretched with water, while the former are dense and of small size. (From CiESiEiiSKi, '72.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance o
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