Longevity of Seeds. 405 



coverintr without directly affecting the palisade cells. Such 

 seeds swelled in water, and microscopic examination showed 

 that the ends of the palisade cells were quite intact, but had 

 separated from each other, as shown in Fig. 79 [7]. From this 

 it would api>ear that the outer niembrane is instrumental in 

 conferring impermeability on the seed, though not directly re- 

 sponsible for it, as is the case with a true cuticle. It seems 

 probable that it serves rather as a cement substance, by means 

 of which the cuticularised ends of the cells are held closely 

 together, thus forming a barrier through which the water 

 cannot jjenetrate, and as soon as the membrane is removed, the 

 ends separate, and the water passes in betw^een them. 



In all the foregoing cases the treatment also took place at 

 average room temperature — i.e., 12 to 15 deg. C. 



Albizzia (Acacia) lophantha. — The seeds used for this test 

 were not fresh material as was the case with the other five, but 

 old seeds which had remained buried in the soil for at least 

 twenty-three years. They proved to be remarkably resistant to 

 the action of sulphuric acid, and to have retained their full 

 power of germination. I obtained also some fresh seed of the 

 same kind, for purposes of comparison, and this was not only 

 considerably less resistant, but had a much lower percentage of 

 germination. This is explained by the fact that, during the 

 time the seeds remained buried in the soil all the non-germin- 

 able and less resistant seeds had either decayed or germinated, 

 and those that were left represented the naturally selected good 

 seeds of many seasons. The degree of resistance varied to an 

 astonishing degree ; some of the seeds swelled after one to five 

 hours in acid, about 38 per cent, required 40 hours' treatment 

 to make them swell, while 6 per cent, only swelled after an 

 application of over 80 hours. The average temperature during 

 the treatment was 12 deg. C.^ I made a second test, keeping 

 the acid at 30 deg. C, and found that about 80 per cent, swelled 

 after 7^ hours in acid, and the remainder at the end of 10^ 

 hours. This was the only case in which I tried the effect of 

 using sulphuric acid of different temperaturee. Many seeds, 



1 Accordinji to Hiltner (Arbeiten aus der Biolog. Abteil f. Land w. Forstwirtsch am 

 Kaiserl. Gesundheitsamte Bd. III., p. 25), 1903. "Seeds of A. lopliaiitha required lO-l.l 

 hours ill H2SO4. to make them permeable — no temperature mentioned." 



