146 STUDIES IN SEEDS AND FRUITS 



return to their original weight. On this point the principle of 

 Berthelot, discussed in Chapter VII, throws a flood of light. 



(14) The contrast between impermeable and permeable seeds is 

 further illustrated by their different modes of responding to high 

 temperatures under the protection of their coats. Whilst impermeable 

 seeds, when exposed for two hours to a temperature of 100 to I io c C., 

 lose only about 25 per cent, of their water-contents, permeable seeds 

 that have completed their drying in air lose under these conditions in 

 the oven as much as 75 per cent. (p. 138). 



(15) But the contrast is extended when we compare the results of 

 their efforts to regain from the air the water lost in the oven. The 

 impermeable seed makes a very slight effort in this direction ; and 

 whilst its coats quickly resume their impervious character, the seed 

 doggedly refuses to make any response to the variations in the 

 atmospheric humidity and adds nothing to its weight when placed in 

 water. On the other hand, after the oven test the permeable seed 

 slowly regains from the air the water lost, but so gradually that weeks 

 are taken up in the process, the original weight being ultimately 

 attained subject to the ordinary hygroscopic reaction. The return of 

 the air-dried permeable seed to its original weight after the heat test is 

 a point of importance, since seeds that have not completed their drying 

 in air fail to reach their original weight, a critical distinction discussed 

 in detail in the chapter on Hygroscopicity (p. 139). 



(16) The great resistance which a seed protected by impermeable 

 coats is able to ofter to extremes of moist and dry heat ranging up to 

 1 00 C., and to alternating dry and damp conditions, is shown in the 

 behaviour of impermeable seeds of Canavalia obtusifolia. Kept under 

 the strain of a great variety of extreme conditions for seven weeks, 

 seeds with coats intact did not vary i per cent, in weight, whilst those 

 with punctured coats varied as much as 7-5 per cent. (p. 140). 



