Longevity of Seeds. 197 



f^ives widely different percentages for the same seeds according 

 to whether they were germinated in a gi'een-hoiise or in a control 

 chamber, the percenti\iges being sometimes much higher and 

 sometimes much lower in the latter case than in the former. 

 This oan only be due to inaccurate Avork, and the same explana- 

 tion possibly applies to the fact that the year-old air-dried seeds, 

 and in some cases the buried ones, gave higher percentages than 

 the same seeds when originally tested. Duvel took no precau- 

 tions to meet the case of hard, non-swelling seeds, and in some 

 oases at least the seeds in question were ones with hard or more 

 or less cuticularized coats. The weathering, softening, or abra- 

 sion of the seed coats (or fruit-walls) by rendering them more 

 permeable to water and oxygen, might easily caiuse a higher 

 percentage germination of the older seed. This is, however, not 

 a true case of after-ripening, which is strictly a vital process 

 taking place in the living contents of the seed. In other cases 

 where seeds are supposed only to germinate after being dried, 

 the drying appears to act by causing cracks to appear in an 

 otherwise impermeable integument. Crocker (I.e.) has, in fact, 

 shown that several cases of delayed germination supposed to be 

 due to after-ripening are really due to the impermeable proper- 

 ties of the seed coats. Thus Asci/ris amarairthoides has di- 

 morphic. seeds, c insisting of (a) flattened winged forms with per- 

 meable ooats, which germinate readily, and (b) rounded forms 

 with impermeable coats, which germinate only after considerable 

 delay unless the seed coats are broken. In the case of Abut Hon 

 Avireiniiae and C'henopodium alhum, the seed coats of otherwise 

 similar seeds are unequally permeable, some swell at once, others 

 only after prolonged soaking. The same is the case with the 

 seeds of the Leguminosae in the foregoing lists. The " hard "' 

 seeds are usually smaller than the readily swelling ones, but this 

 is simply because they are drier ; when swollen the sizes are 

 approximately the same, and in the act of swelling many of the 

 '' hard ' Acacia seeds often increase by 2 to 4 times the original 

 bulk. 



In the case of Playitayo major. F. Ruegelii, Thlaspi arvense 

 and Avena fatua, the delay in germination is also due to the 

 seed coats, and oan be overcome by removing or breaking them. 

 When they are intact the minimum temperature for the germina- 



