226 



STUDIES IN SEEDS AND FRUITS 



gain in weight very slowly, or they may remain absolutely un- 

 changed, or they may acquire a stable weight subject only 

 to hygroscopic variation about a mean. A period of a few 

 weeks or of two or three months is usually sufficient for the 

 completion of the process of drying in free air. For most 

 permeable and impermeable seeds this is quite enough, as 

 is shown in Note 12 of the Appendix, though longer periods 

 may be occasionally required, as in the case of Mammea 

 americana^ specially referred to in Note 6, where the drying 

 was continued for about a year. 



Speaking generally, my data show that impermeable seeds 

 can retain their weight unchanged for at least four years, 

 excluding such small variations, amounting in the case of 

 Entada scandens to only about 2W0 °^ ^^^ ^°^^^ weight, 

 which are mainly instrumental in their nature. On the other 

 hand, during the same period permeable seeds, assuming what 

 we may call "the hygrometric state," display a variation in 

 weight of 2 or 3 per cent, around a fairly constant mean 

 in response to the varying humidity of the atmosphere. 

 Variable seeds again behave in an intermediate but change- 

 able fashion, the result of their varying proportions of 

 permeable and impermeable seeds, sometimes approaching 

 one type, sometimes the other, but usually holding a half- 

 way position and exhibiting a small hygroscopic range of 

 1 per cent, or less. 



Coming first to the impermeable seed, we are justified, I 

 think, in believing that as long as it preserves the same weight 

 and is non- hygroscopic it retains its vitality. It enjoys 

 complete immunity from the dangers of the hygroscopic 

 reaction, which, if continued through years, must, as M. Jodin 

 observes, ultimately induce molecular changes and lead to 

 the loss of germinative capacity in the case of the permeable 

 seed. The impermeable seed, on the contrary, as long as 

 it is true to its original weight, is exposed to no such risk. 

 However, Professor Ewart, in his paper on " The Longevity 

 of Seeds," contends that " even when perfectly inert a macro- 



