The Biology of Senescence 



The 'senescence 5 of some lines of plants in vegetative propa- 

 gation apparently depend on the accumulation of exogenous 

 viruses which hamper vigour (Crocker, 1939) — other agricul- 

 turally important varieties have been propagated vegetatively 

 for years or centuries without deterioration. The accumulation 

 of exogenous viruses itself raises interesting questions in regard 

 to the possible accumulation of other, endogenous, intra- or 

 extranuclear self-propagating materials. 



Not all senescence or degeneration in clones, however, can be 

 put down to the peculiarities of protozoa or to the action of 

 viruses. A striking example of such a degeneration has been 

 studied at Oxford by K. G. McWhirter, to whom I am much 

 indebted for his unpublished observations on it. This is the con- 

 dition called 'June Yellows', which affects strawberry plants 

 propagated by runners, and impairs the formation of chloro- 

 plasts. It appears simultaneously in all plantations of a clone, 

 even when they are geographically separated, and progresses in 

 jumps, all the plants of the same clonal (but not individual) age 

 passing synchronously from stage to stage. Usually in the end 

 the clone dies out. The condition cannot be transmitted to adult 

 plants by grafting. Transmission to seedlings is ambilinear 

 through both egg-cell and pollen. In the progeny of crosses 

 between clones at different stages of degeneration it is matro- 

 clinous: seedlings of very degenerate 'mothers' deteriorate most 

 rapidly. As a clone degenerates, the tendency to transmit 

 'yellows' to its offspring increases. The factor or factors remain 

 latent in some clones, but 'yellows' may appear after varying 

 intervals in some of the selfed or crossed seedlings obtained from 

 these clones, thus showing a latency reminiscent of that of the pre- 

 sumed oncogenic plasmagenes. This similarity has been pointed 

 out before (Darlington, 1948; Darlington and Mather, 1949). 



The behaviour of this degeneration is like that of a mutation 

 which is in part cytoplasmically controlled. Such conditions are 

 characterized by a lag-phase, by simultaneous appearance in all 

 the members of a clone, non-infectivity, passage through a series 

 of stable phenotypic stages, and interaction with growth and 

 reproductive hormones. In some of McWhirter's material, 

 'yellows' appeared to be aggravated during the flowering period, 

 although it may occur in seedlings long before flowering, 



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