Mechanics of Auxin-induced Growth 327 



DISCUSSION 



Dr. Bennet-Clark: The suggestion that has just been made that 

 growth is only enlargement and that division in two is not growth 

 seems to me quite incorrect. Cell division involves complex differen- 

 tiation. Cell enlargement can be an osmotic intake of water involving 

 no differentiation, as for example, recovery from wilting. 



The growth processes that we have been discussing are those 

 promoted by so-called auxins or growth hormones, the very existence 

 of which was first revealed by studies of tropisms. I would like to re- 

 fer to the behavior of two geotropic mechanisms which seem to be 

 remarkably different. The first is that of the first internode or meso- 

 cotyl of Zca mays which is a desirable research object as on geotropic 

 stimulation it bends very sharply with small radius of curvature, 

 which means that the length and volume of cells on the convex side 

 are markedly larger (some 30 to 40 per cent) than those on the con- 

 cave side and consequently analysis of the differences between them 

 is facilitated. This bending is caused by different growth rates of 

 the t\\'o sides of the mesocotyl. 



The other quite distinct behavior is that of a grass node. When 

 placed on its side, the lower side expands and the node becomes 

 sharply bent upwards because these lower-side cells expand about 

 200 to 300 per cent in length and volume. Is this to be called growth? 

 They do not have any normal capability of growth like mesocotyl 

 cells, and expand only in response to the gravitational stimulus. 



Some of the earlier workers in the last decade of the nineteenth 

 century showed that a geotropic stimulation of stems (inflorescence 

 axes) resulted in increase of extensibility of the tissue on the lower 

 side. A given bending moment caused greater deflection away from 

 the lower side than that away from the upper side. With a grass node 

 this situation is reversed: it bends more readily towards the upper 

 than to the lower side. This is associated with the fact that cells on 

 the lower side are more turgid than those of the upper side, and they 

 are consequently more rigid. The increase in their osmotic pressure 

 "blows them up" and causes the expansion and the rigidity. 



Structurally these cells are very remarkable: The tissue has a 

 concertina-like appearance and the cells have relatively thin walls 

 at right angles to the long axis of the node and very thick walls oblique 

 to this long axis. On expansion, which is sometimes as much as a 

 fourfold extension, these thick oblique walls pull out like the oblique 

 folds of a concertina. This can hardly be described as growth. It does, 

 however, involve factors important in growth; the first is the increase 



