THE FRUIT AND SEED-DISPERSAL 333 



The Orchidaceae are extreme cases of productivity : the estimates 

 of seed-production in them are as follows : 



Such figures convey little more than a general impression of vast- 

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

 the actual requirement to make up directly for losses by death. There 

 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. Such 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 seedling. 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. 



On the other hand, carefully recorded cases under the favoured 

 conditions of cultivation illustrate how prolific plants may be, even 

 where the productivity in number of seeds is not specially high. A 

 good example is seen in the " Marquis " Wheat, which was derived 

 from a single head in 1903, as a result of hybridisation. It has since 

 been spread through Canada and the United States. In 1918 it was 

 sown on 20,000,000 acres of land, and yielded some 300,000,000 bushels 

 of grain. So wonderful a result in a cultivated plant under control, 

 in a limited number of years, illustrates the effect of a geometrical 

 ratio of increase, such as might often be possible in Nature, if the 

 circumstances were equally favourable. 



