92 Trees, Stars, and Birds 



loads of these delicious nuts to Eastern markets, and 

 hardy varieties of the tree are beginning to be grown 

 in our Eastern states. Nevertheless, 

 some thirty million pounds of walnuts 

 a year are brought to the United States 

 from France, Italy, and other foreign 

 Jf\ countries. 



Pecans are the most important nuts 

 in the market that are produced by 

 FIG. 54. Ceils from trees native to America. Pecan 

 l h V h f t? v P i, Can ; orchards have been planted in many 



Note the thick walls of * 



these cells, which give places in the South Atlantic and Gulf 

 the hardness to the s t a t es an d produce nuts much superior 



tissues. 



to those of the wild trees. Why are 

 these nuts more in demand than those of other species 

 of hickory trees? 



The hard tissues of plants. Examine a fruit of a 

 walnut or hickory tree. The shell is dense and hard. 

 These hard parts of plants are formed by the thicken- 

 ing of the walls of the cells that compose them. Thus 

 in the shells of some nuts the protoplasm of the cells 

 builds up the walls about them until the cavities of the 

 cells are almost filled. This makes a tissue which has 

 little air space in it, and is in consequence very hard. 

 Not only in seeds and fruits, but also in the outer 

 layers of many plants and in other places, are these col- 

 lections of hard cells found. Where their walls are very 

 thick they are called " stone cells." 



