SEINFOBCED FSUITS 



287 



338, It is commonly assumed that these rein- 

 forcing structures are the result of adaptation,— 



that is, that they afford 

 protection to the seed. 

 It is no doubt true that 

 they protect the seed, 

 but it is by no means 

 evident that this protec- 

 tion has been developed 

 for the purpose of pro- 

 -K; tection, or that it is 

 primarily useful to the 

 plant. The walnut fruits 

 are so impervious that 

 the seeds cannot grow 

 unless under the most 

 favorable conditions. That is, there can be evi- 

 dence adduced to show that 

 these indehiscent husks and 

 hard nuts are a detriment to 

 the plant, as well as a bene- 

 fit. Again, it is difficult to 

 explain, upon the adaptation 

 hypothesis, why the husks of 

 hickories are dehiscent and 

 those of walnuts are not, or, in 

 fact, why these nuts have husks at all, whereas 

 the many other fruits do not. Nor is it easy to 



Fig. 293. 

 Chestnut bur. 



Fig. 294. 

 Section of an apple. 



