Senile Changes in Leaves of Certain Plants 327 



and phloem tends to keep the supply up to the demand. In most 

 perennials conduction occurs largely through tissues that have been 

 specialized but httle longer than the leaves. In the plant, as in the 

 animal, the organs present have a capacity above the ordinary require- 

 ments of hfe, and therefore senile degeneration in function can continue 

 for some tune before the point is reached at which the efficiency drops 

 below the daily requirement. This interesting point will be considered 

 again under the section dealing with carbohydrate production. 



If meristematic tissue does undergo senile deterioriation, then cambium 

 must produce less efficient conducting tissue as time goes on, and 

 ultimately its efficiency will sink to a point at which the leaves will begin 

 to suffer from lack of supphes as well as from their own senile degeneration, 

 and senile death will come apace. In these experiments, however, the 

 older vines, with four exceptions, were in the stage of vigorous maturity 

 and evidently had not reached the stage of inefficient transport. The 

 idea that the differences in size of vein islets is primarily due to difference 

 in efficiency of transport in older parts of the plant must therefore be 

 definitely abandoned. 



The possibihty of the production of toxic substances by the old parts 

 of the root and the stem, and their being carried up to poison the meri- 

 stem and the leaf, is a fascinating conception. In the theory of senility 

 which Metchnikoff (1908) has based on the autotoxin conception, the 

 source of the toxin is said to be in man the large intestine and in annual 

 plants the flower; while in order to meet the conditions of the present 

 problem with perennials, the old wood and bark must be considered 

 the guilty organs. 



It is possible that autolytic changes occur in the cells of the old 

 speciahzed tissues of the wood and the bark with the production of various 

 substances. It is possible also that some of these substances may escape 

 from the region of their origin, enter the transpiration stream and be 

 carried to buds and leaves, and accumulate there in sufficient concentra- 

 tion to affect the meristematic tissue. But there is no direct evidence 

 that toxic substances are produced or that such poisoning occurs. On 

 the contrary, the evidence from the studies of venation here described 

 is directly opposed to the theory that the changes in size of the vein 

 islets are due to contact with toxins produced from the old speciahzed 

 tissues of the plant. In the mature leaves of Vitis vulpina growing 



