THE FRUIT AND SEED-DISPERSAL 297 



The Orchidaceae are extreme cases of productivity : the estimates 

 of seed-production in them are as follows : 



Such figures convey Jittle more than a general impression of vastness : 

 but evidently the number of germs produced is far in excess of the 

 actual requirement to make up directly for losses by death. Tiicre 

 is in fact an immense margin, which may be regarded as a very 

 efficient reserve to meet all the contingencies involved in the establish- 

 ment of the germ till it reaches propagative maturity. Sucii a 

 reserve is necessary, for the risks of youth are great. Many seeds 

 fall victims to the predatory attacks of animals, which naturally 

 divert to their own uses the food-stores laid by for the germ. Many 

 never reach a situation fit for their germination. Many young 

 plants are killed off almost at once by unfavourable conditions, 

 such as unsuitable temperature, or drought, or unseasonable changes 

 while in the defenceless condition of the seedhng. Competition with 

 the same or other races of plants destroys others. Fungal attack 

 also takes its toll, and especially in the seedling state. But notwith- 

 standing the number, and insistence of these risks, an overplus 

 remains in any surviving species. This not only keeps the race in 

 being, but in most cases provides for its spread into fresh areas, 

 where, however, it is liable to be checked by various limiting factors. 

 Moreover, the large numbers, and the competition which necessarily 

 follows, provide material for Natural Selection to work upon : and it 

 is the fittest that will be the most likely to survive. 



Latent Period. 



■ After seeds are shed and distributed they usually undcrii.. .1 /.'.lu 

 of rest. During the autumn and winter of temperate climates they 

 become buried in the soih They naturally fall into chinks .uui 

 crannies, and are often covered by rotting leaves ; they arc also washed 

 into the soil by rain, or drawn below by the restless activity of earth- 

 worms, or covered by their castings. But some work their own way 

 into the soil by hygroscopic movements, as in Avena, or Sltpa. or 

 Erodmm. A few even bury their fruits as they mature by geotropic 



