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BOTANICAL GAZETTE 



themselves; that is, vegetative propagation does not bring about rejuvenes- 

 cence in this respect. The rate of decrease in the size of the vein islets is high- 

 est in the earlier years of life of the vine and decreases in later years, and when 

 plotted as a curve shows a decrease in steepness and approach to horizontal 

 with advancing age. The identity of type of this curve with the curve of 

 decrease in pulse rate in human beings and the decrease in rate of growth in 

 guinea-pigs is strikingly shown by graphic comparison. 



These observations on Vitis vulpina are supplemented by less complete 



trees, including species of Salix, Castanea, Quercus, Tilia, Ulmus, Carya, Acer, 

 Platanus, and Fraxinus, all of which lead to the same conclusions, as do also 

 observations on several pedigreed varieties of grape which have been propagated 

 by cuttings for known different lengths of time. Other age changes determined 

 in the leaves of Vitis are decrease in rate of C0 2 production, decrease in imbi- 

 bition of water by powdered leaves, decrease in acidity, increase in number and 

 decrease in size of stomata, and probably a decrease in size of palisade cells 

 and an increase in the proportion of cytoplasm to nucleus. 



cultivated fruits and other plants propagated by vegetative means, and then, 

 after criticism of some of the various theories of senescence based primarily on 

 zoological data, Benedict concludes that a decrease in permeability of the cells 

 will best account for the observed facts. With this decrease in permeability 

 of the photosynthetic cells, diffusion of water and salts through them is retarded, 

 and the results obtained by various investigators indicate that lack of water in 

 the cells of the leaf stimulates the production of veinlets. He points out, how- 

 ever, that this decrease in permeability with advancing age is itself dependent 

 upon the properties of colloids, and that the fundamental factors of senescence 

 are to be found in the properties of the protoplasmic colloids. 



The reviewer welcomes this paper, not only for its scientific and practical 

 significance, but also as affording in general confirmatory evidence for his own 

 conclusions based on an experimental study of certain simple animals, and for 

 certain suggestions concerning senescence and rejuvenescence in plants. It 

 seems worth while to point out, however, that changes in the meristematic 

 tissue, such as Benedict has discovered, are not the only age changes in the 

 plant. The cells which differentiate from the meristematic tissue undoubtedly 

 grow old much more rapidly than the meristematic tissue. The changes in 

 the meristematic tissue of Vitis which determine the decrease in size of the vein 

 islets must be at most only the earliest stages of senescence. Differentiation 

 in any appreciable degree is not yet present in this tissue, but that is no reason 

 why it may not become an important factor in more advanced stages of senes- 

 cence. Protoplasmic differentiation is undoubtedly of less significance in this 

 respect than in animals, where its relation to senescence is evident. 



