432 THE BIOLOGY OF FLOWERING PLANTS 



The seedling drops straight from the tree into the mud, and 

 is, as it were, dibbled in ; fixation occurs by the rapid 

 development of a shallow root system. 



§ 4. Germination — Conditions 



When the seed is finally capable of germination, this can 

 take place only if external conditions are favourable. Most 

 important are water and oxygen supply and temperature, 

 though we may look on light as a necessary condition for 

 the seeds showing the fourth type of dormancy. 



The amount of water absorbed by the seed may be very 

 large. In the wheat it ranges from 45 to 60 per cent, of 

 the dry weight, in the maize from 35 to 40 per cent., in the 

 pea from 84 to 106 per cent. The absorption is at first 

 entirely due to the imbibition of the colloids of the embryo, 

 endosperm, and seed coats ; later, when the cells have 

 become vacuolate, or rather the collapsed vacuoles have 

 filled, osmotic suction sets in. Though, as we have seen, 

 seeds can absorb water from quite dry soils, it does not 

 follow that germination will occur in such conditions. Even 

 when the supply of moisture is sufficient to saturate the seed 

 and permit the extrusion of the radicle, it may not enable 

 the seedling to establish itself. The decisive effect of a 

 liberal water supply is clearly seen in the great crop of 

 weeds which spring up after summer showers following a 

 dry spell ; or still more vividly in the sprouting of desert 

 ephemerals as soon as the rains set in. 



A special case of much interest is that of the coconut 

 in which germination takes place very soon after ripening 

 at the expense of the store of water in the seed, evaporation 

 of which is lessened by the thick husk of coir within the 

 pericarp. The leaves appear first, and several may put 

 forth before the adventitious roots are produced in wet 

 weather, and enable the plant to establish itself. In this 

 fruit, which retains its viability only for a short time, it is 

 important that germination should be able to occur in the 

 dry seasons — when the largest crop ripens — and without the 



