SEED. GERMINATION 



i 



and left, to a curved body which lies between them. t ol tin- 



compressed between the Seed-leaves; it is the leafy bud or plumule, 

 which is to grow into the shoot of the seedling. Pointing in t he- 

 opposite direction to this is the first root or radicle, the conical tip 

 of which is close to one end of the hilum. The parts thus recognised 

 are present in all normal seeds of Dicotyledons, which take their name 

 from the paired Seed-Leaves. But the form and proportions of i 

 seed and of the germ may vary in different plants, and certain 

 additional tissues may in some cases be present. The Seed-L 

 of the Bean are fleshy in texture, and are stored with materi 

 which serve on germination for the nutrition of the other parts of 

 the germ. 



The conditions necessary for the germination of a living seed 

 constructed are : (i) the presence of moisture ; (ii) free access to atmo- 

 spheric air ; and (hi) a suitable temperature. The ordinary conditions 

 of spring-time would meet these requirements, if the seed were buried 

 in an open porous soil. For the soil would be moist, and the air would 

 be free to penetrate its pores ; while the rising temperature of the 

 season w T ould meet the third requirement in the case of seeds of 

 temperate climates. 



Supposing these conditions to have been fulfilled in the case of 

 a living Bean-Seed, germination ensues. The first external change 

 is the rupture of the seed-coat at a point close to the end of 

 the scar. Through this the pale young root protrudes, and as it 

 elongates its conical tip at once penetrates vertically downwards 

 into the soil. When the root has attained a length of several inches, 

 a curved shoot emerges through the slit between the bases of the 

 two seed-leaves. Clearly this is the result of growth from the leafy 

 bud seen between the cotyledons in the seed. This shoot turns 

 upwards, and soon projects above the level of the soil. Growth is 

 thus seen in both parts, the material required for the enlarging 

 tissues being supplied from the fleshy seed-leaves. This is used 

 in building up the enlarged root and shoot. No drying-up will 

 make the root or shoot shrink back to their original size or form, 

 nor can the material drawn from the seed-leaves be by any means 

 replaced. Such growth is an irreversible process, as is all growth 

 in Plants. 



That the root points downwards and the shoot upwards is not a 

 haphazard result. Experiments with seeds placed in various positi 

 in the soil show that the behaviour of these two parts of the seedling 

 is constant, and suggest that it is a response to some external infll 



