Discussion 213 



number of polyphosphates besides the relatively simpler ATP/ADP 

 system found in animal tissues, and it is a problem of some complexity to 

 separate these from one another in analytical determinations. We have 

 tried the methods developed for animal tissues, such as barium salt 

 precipitation, without success. 



Villee: The other thing I would like to mention is that in point of fact 

 you measure CO a production, and it might be a little dangerous to call 

 this "respiration ". Of course, in plants you do have this special problem 

 of the simultaneous production and utilization, so to speak, of oxygen 

 and carbon dioxide, and it is very difficult to separate respiration from 

 photosynthesis. 



Yemm: Yes, but of course all the respiratory data are obtained in the 

 dark. The two processes are separated in that way. In point of fact, 

 with barley and some of the other leaves, respiratory quotients have 

 been measured. The respiratory quotient of the yellowing leaf tends to 

 be less than one, so that if one considered oxygen uptake it would show 

 an even more striking increase during senescence than C0 2 . The respira- 

 tory quotient of less than one, I think, just reflects the fact that other 

 substrates such as the carbon skeletons of amino acids are oxidized in 

 ageing tissues besides sugars. 



Villee: Yes. That is what I was leading around to, because the CO a 

 may come from many different things. And, of course, the oxygen 

 which leaves the plant as CO a does not necessarily come in as gaseous 

 oxygen. 



Williams: I would like to go into the question of ageing in general. 

 Dr. Yemm seems to be dealing with the ageing of transient tissues but 

 as far as I can gather it seems to be rather for the benefit of the per- 

 petuation of the plant, does it not ? For instance, in a thing like barley, 

 the products are transferred to the seed. I gather, too, that in a de- 

 ciduous tree you are presumably using up metabolites or storing them 

 for the next season. Now, in that sense I should have thought that this 

 was more like the metabolism of the muscle in a salmon, for instance, 

 in the breeding season which is presumably for the benefit of the salmon 

 rather than an ageing process which is detrimental as it is in most animal 

 systems. What happens, for instance, in perennial garden plants which 

 are not everlasting? Is there, in fact, a reproductive life and then after- 

 wards is there a senile stage in those sorts of plants where reproduction 

 does not persist but the plant lives for some years before dying, and in 

 those cases when the plant does die are there the same sort of changes ? 



Yemm: Well, your first point: I think that senescence of the leaves 

 is a part of the ontogeny of the plant as a whole, and I think it is perhaps 

 desirable that senescence in animals should be regarded in the same way. 

 There is no such thing as an isolated senescent tissue — it seems 

 necessary to consider it in relation to what is happening in the rest of the 

 organism. 



The second point : I do not think there is true senescence of whole 

 plants which propagate themselves vegetatively. It was for a long time 

 thought that this was the case in potatoes, strawberries and other plants, 

 but it has turned out in many cases that it is the tendency to develop 



