462 THE BIOLOGY OF FLOWERING PLANTS 



leaves, measured by the area of the mesh between veins, 

 gets less as the plant ages, and that this is an indication of 

 senility. Ensign (1921), however, disputes Benedict's obser- 

 vations. We have seen that in the case of an annual plant, 

 the sunflower, there is an undoubted decrease in the 

 metabolic activity of the protoplasm as the plant grows 

 older. It remains to be seen whether any definite evidence 

 can be obtained that the same thing takes place in the 

 successive years of the life of a perennial. A decrease in 

 respiring power has been found in older leaves of the olive 

 by Nicolas (19 18), and in leaves of older trees of the olive 

 by Ruby (191 7), but the data are scanty. 



On the other side we have the fact that many cultivated 

 plants have been reproduced from prehistoric times by 

 cuttings only, and, in the banana, for example, which sets no 

 seed, show perfect vigour. The sweet- flag, Acorns Calamus, 

 was introduced into Europe from Asia over 500 years ago, 

 and since its introduction it has multiplied vegetatively 

 and spread vigorously ; no seed has been set, for the insects 

 necessary for pollination are absent. Similarly the water 

 thyme, Elodea canadensis, was introduced once or twice 

 into the waters of this country in the 'forties of last century ; 

 only the female plant grows here, and the enormous vigour 

 of its spread has been due entirely to new branches breaking 

 away from old stems. In any locality, after a few years' 

 growth, its vigour seems to decline, but this looks more like 

 the effect of external conditions than of internal change. 

 Many native plants are reproduced almost or entirely by 

 propagation — the lesser celandine, the periwinkle, and some 

 meadow grasses and fescues ; and we may recall the case 

 of the parthenogenetic hawkweeds. The evidence is 

 ambiguous, but on the balance it seems to point to the 

 possibility of a very long and, perhaps, in some cases, 

 indefinite activity of the plasma, which originated in a single 

 reproductive cell and has since split up in innumerable 

 growing-points, without the intervention of a rejuvenating 



process. 



Causes 0! Death.— The cause of death would thus 



