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 differential imbibition. Among slow tropisms are hydro- 

 tropism and rheotropism, 

 which are probably due 

 largely to differential as- ep 

 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 water, while those of FlG . 140 . - A section of a tropic radicle taken in 



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 measurements 

 of the dimension of the cells in the curving region, compared 

 with normal cells, is given by CIESIELSKI ('72) as follows : 



the plane of curvature, at the region sy, Fig. 

 10(). ep, epidermis ; rp, parenchyma ; gbs, 

 sheath of the fibro-vascular bundle ; Izb, fibro- 

 vascular bundle ; h, wood-cells ; .(/, vessels. 

 Those cells which lie next the nadir () 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 CIESIELSKI, 72.) 



