REPRODUCTION AND DISPERSAL 909 



legume family. In experiments made with dried 'seeds of considerable 

 age (none under 25 years old), involving over 500 species belonging to 

 30 families, those of 23 species distributed among 4 families proved 

 viable, 18 of these being among the legumes; in these experiments the 

 oldest viable seeds had an age of 87 years. Other experiments indicate 

 the maximum retention of viability by legume seeds to be 150 to 250 

 years. Other long-lived seeds are those of the water lilies, the mallows, 

 and some of the mints. 



In the phenomena of longevity there are three features of special in- 

 terest: the status of the Irving cells during this long period, the features 

 of the seed that cause the retardation of death, and the nature of the 

 factors that ultimately cause death. There have been two theories con- 

 cerning the status of the living cells, namely, that they manifest very 

 slight respiratory activity, and that they are in a state of suspended ani- 

 mation. 



It is not possible at present to determine which theory of cell life is the more valid, 

 but recent experiments seem to give strong support to the theory of suspended ani- 

 mation. There is no adequate evidence of respiratory gas exchanges nor of any 

 other metabolic activity in dry seeds; the very small gas exchanges that have been 

 noticed are quite as characteristic of dead seeds as of living seeds, and in the latter 

 they are fully as prominent in the dead testa as in the embryo. Furthermore, the 

 theory of suspended animation best accounts for the wonderful resistance of seeds 

 to extreme temperatures; indeed, seeds can endure a temperature so low that 

 activity of any kind under such conditions seems quite impossible. The likeli- 

 hood that activities may take place in seeds has been suggested from the fact that 

 recently matured seeds of certain species germinate poorly, if at all, while, without 

 any obvious structural change or physiological activity, they germinate readily 

 after a lapse of some months. However, this theory has become less tenable in 

 view of the discovery that differences in the germinability of seeds are due chiefly 

 to changes in the permeability of the dead seed coat. In any case the life processes 

 of seeds, if present, are intracellular and anaerobic and are exceedingly minute in 

 amount. 



It is practically certain that the chief feature of seeds which retards 

 premature germination and facilitates longevity is the impermeability 

 of the enveloping coat, especially of the testa. In many instances the 

 seed coats of desiccated seeds have been found to be nearly impermeable 

 to water and to gases; the most impermeable of such envelopes are 

 those of legume seeds, which, as has been noted, are the longest-lived of 

 all. The compact structure and the low water content of seeds are un- 

 favorable to activity, and hence facilitate longevity. Certain short-lived 



