Discussion 211 



Yemm: Most of these leaves are not deciduous. They have quite a 

 long life and are commonly relatively resistant leaves of the type of 

 Cherry Laurel. 



Montagna: The factor of light is also rather important. I made some 

 years ago what I thought to be a very perspicacious observation when 

 I noticed that when in the autumn leaves of maple trees become yellow, 

 all the leaves which face towards street lights remain green! When 

 I confided this to my botanical friends they told me that everybody 

 knows that! At any rate, in these deciduous plants, the leaves near 

 the light remain much longer than those away from the light. 



Yemm: The evidence from more exact studies under controlled 

 conditions is a little conflicting! Certainly with old leaves you get very 

 little retardation of protein breakdown by keeping them in the light, 

 compared with in the dark. 



BourlUre: Has any study been made on the ageing of seeds? What 

 happens, for instance, in seeds which retain their germinative power for 

 a very long time ? 



Yemm: There are certainly gradual changes in seeds. The longest- 

 lived seeds, which may survive for something like 200 years, do, never- 

 theless, gradually lose their ability to germinate. And seeds which are 

 not quite so resistant have been shown to have quite appreciable res- 

 piratory activities; they are not entirely dormant structures, and there 

 are biochemical changes in certain cell constituents. One of the quite 

 interesting suggestions which has recently been made is that there is 

 a gradual oxidation of glutathione and possibly other sulphydryl com- 

 pounds; when this has reached a certain point then the seed may no 

 longer possess the capacity to germinate. 



Krohn: I wonder if Dr. Yemm would see any similarity in the life of 

 the antlers that we have heard about this morning and the life of his 

 leaves. Do they fall off in the same sort of way? What I particularly 

 wondered was whether the petiole, which joins the leaf on to the rest 

 of the plant, atrophies there and shrinks up first and then the changes 

 in the leaf are secondary to that ? 



Yemm: No. The process of leaf abscission is quite an active one. 



Krohn: As in the antler? 



Yemm: Yes. It involves the production of an abscission tissue. Cell 

 division begins in the petiole of the leaf and leads to the formation of a 

 layer of abscission cells which ultimately cut off the leaf from the rest 

 of the plant. But, of course, this is not an essential feature of senescence 

 because in many annual and other plants the leaf undergoes senescence 

 while still attached to the plant. 



Krohn: The other thing I wanted to ask was, do you think that in- 

 fections with virus diseases and so forth, which are clearly persistent 

 in the leaves, cause an accelerated ageing of the leaf? They obviously 

 make them go yellow. 



Yemm: Well, I think that is a part of the same general process. Virus 

 infection, generally speaking, probably leads to a diversion of protein 

 synthesis from the synthesis of normal cytoplasmic elements in the cell 

 into the special viral elements. In other words, protein synthesis is 



