THE LIFE AND GROWTH OF SEEDLINGS 21 



some time for the seed to absorb as much water as it can take up. 

 When a broad bean, or a pea, or, in fact, any seed is soaked, 

 the gradual absorption of the water is evident from the stretching 

 of the seed-coat. When dry, this is more or less wrinkled, but 

 after three or four days the seed-coat becomes tense and splits, 

 allowing the radicle to protrude. As seeds swell, they exert 

 great force ; it is stated that a large mass of swelling peas may 

 lift 100 Ib. 



(iii.) TEMPERATURE. Seeds require Warmth. Seeds do not 

 germinate without a certain degree of warmth. A convenient 

 temperature is about that of an ordinary living room ; any- 

 thing between 21 C. and 35 C. will answer for the majority 

 of seeds. Above a certain temperature, seeds will not germinate. 

 Similarly, if exposed to too great cold, as, for instance, freezing- 

 point, germination does not take place. It is advisable to keep 

 germinating seeds in an equable temperature and rather moist 

 air. One reason for planting hedges round fields is to protect 

 the young crops from the strong, often very cold, March winds, 

 which might injure the seedlings irreparably. 



(iv.) Germinating Seeds require Food. It is by the absorption 

 of water and the food-material contained in it that young 

 seedlings get their food. In the first few days of germination, 

 seeds, like the bean, with fleshy cotyledons feed on the food 

 contained in the cotyledons. In the case of seeds of rapid 

 growth, like the radish and mustard, the radicle very soon 

 develops root-hairs which absorb food-material from the soil. 

 This food-material is taken in with the water. Even when the 

 seeds are grown on damp blotting paper, or on a sponge, a certain 

 amount of food-material will be absorbed with the water, for 

 no water is absolutely pure. From ordinary tap water with 

 mere traces of salts, plants manage to collect large quantities 

 of mineral constituents, and in the case of water left exposed 

 in rain-butts, or open cisterns, the impurities are considerable 

 and furnish food-material for the innumerable algae so com- 

 monly found in it. Seeds planted in soil get their food-material 

 from the mineral substances present in the soil, from the leaf- 

 tould and the manure, all these constituents being dissolved in 

 tter before the root-hairs can absorb them. 



