§ 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.) 



