378 K. V. Thimann and N. Takaliashi 



growth of the controls is slowing down and that this slowing down 

 is due to the deposition of cell wall material which cobalt prevents. 

 According to Busse, the main effect of cobalt is to allow stretching 

 for a longer period of time. I am not personally convinced that this 

 is the whole explanation, although I have not made extensive time 

 studies. As you saw, cobalt exerts its effect on potato slices while 

 the control slices are still growing well. It is true that if you measure 

 it over a very short time, 3 to 4 hrs., there is only a very small cobalt 

 effect, so that while perhaps the effective times may vary with the 

 experimental conditions, there is an increasing effect of cobalt with 

 time. I found two effects of cobalt linked to organic acid metabolism; 

 namely, that in the presence of acetate, the cobalt exerts only an in- 

 hibition and that cobalt prevents the normal excretion of organic 

 acids by coleoptiles and pea sections. I think that the cobalt effect 

 is probably quite complex. 



Dr. Bonner: It might be appropriate to mention some further 

 work which bears on the same final question. It is not otherwise 

 similar to Prof. Thimann's work. We know that calcium is inhibitory 

 to growth of isolated tissue sections. The question is. How does 

 the calcium do it? Wliat does it combine with to bring about this 

 drastic inhibitory effect? Experiments that have been published 

 for some years show that one characteristic of calcium-induced growth 

 inhibition of Avena section growth is that the calcium ions that 

 exert the inhibition are bound exchangeably. The same is true of 

 the bound calcium ions which decrease the mechanical deformability 

 of coleoptile tissue. 



Apparently the calcium ions that make the tissue stiff and not 

 grow rapidly or not bend rapidly are ions that go into the tissue 

 and combine with something; then they stay there and don't come 

 out in water but have to be encouraged out by some other kind of 

 ion such as sodium or potassium. One can measme things about 

 these exchangeably bound ions, the concentration needed to obtain 

 half maximal inhibition of bendability, and the time constants for 

 ecpiilibration of the tissue with the ion. We can also find out, by 

 classical chemical methods, whether the tissue can bind calcium ions 

 exchangeably. We can take tissue and put it in some labeled calcium 

 solution, find out the amount of ion bound by the tissue in such a 

 form that it will not leak out in water but can be exchanged by 

 unlabeled calcium, potassium, or some other cation. In this way it 

 is possible to determine that Avena coleoptile sections, for example, 

 do bind calcium ions exchangeably and they have a certain cation 

 binding capacity that we can calculate in milliequivalents per gram 

 of sections just as if one were a soil scientist. We can also determine 



