Senile Changes in Leaves of Certain Plants 289 



fruit, and are more likely to be attacked by scale and disease. He states 

 further, regarding pear trees, that the epidermis is likely to blister, that 

 the trees are less able to withstand cold, and that the fruit is small, 

 deformed, unpalatable, and with more hard flecks in the flesh. 



The botanist and the horticulturist of to-day add nothing to Zorn's 

 list. The question whether true senility occurs in perennial plants, 

 therefore, is still an open one. The changes spoken of by Zorn and by 

 more recent writers may be effects of disease of external origin or the result 

 of increase in size of the tree, not real results of senility. De Candolle 

 (1876) investigated the question whether the time of leafing was related 

 to the age of the tree, and decided that the age was negligible compared 

 to the climatic conditions. His results in this one particular seem to have 

 been much more widely applied than he probably intended. 



importance and significance of the problem 

 The importance of determining whether there is any real senile change 

 in plants lies not alone in the scientific need for such knowledge, but also 

 in its direct bearing on the long-disputed question regarding the effect 

 of continuous vegetative propagation of seed-producing plants. For if 

 the new growth from which cuttings are made has not been affected by 

 the time that has elapsed since the plant came from the seed, then its 

 tissue is no more senile than seedling tissue; if, however, the meristematic 

 tissue, which has been so actively growing and dividing since it originated 

 in the embryo sac of the parent plant, has itself suffered the senile deteriora- 

 tion that accompanies activity in animal cells, then the tissues arising 

 from this meristematic tissue must partake of its senility. 



The processes of division and growth require considerable expenditure 

 of energy; therefore, if the plant cell is not entirely immune to senile 

 deterioration, the meristematic as well as the specialized tissues should 

 undergo such change. In fact, in the ontogeny of the animal, senile 

 deterioration is most rapid at the time when specialization is least, when 

 the only activities of the cells are division and growth. There is no 

 inherent reason, therefore, why meristematic and cambial cells should be 

 put in a different class from that of specialized cells as regards the pos- 

 sibility of senile degeneration. 



It may be true — no exact comparisons as to this point are on record — 

 that the meristematic cells have a greater proportion of nucleus to cyto- 



