82 



THE PLANT SKELETON 



of nuts and stone fruits. In the latter instances they may form 

 continuous tissues, but in roots, stems and leaves they occur 

 in more or less isolated groups or even singly. They occur in 

 many barks in sufificient numbers to make them notably strong 

 and hard, as in hickory, and they sometimes reenforce the bast 

 fiber tissues by forming firm unions betv^een their separate 

 strands, as in the oaks. They contribute to the tough and 

 leathery character of some leaves, as in Camellia and Olea, 

 for example, where they occur scattered among the mesophyll 

 cells, to which they, indeed, belong morphologically. Small 

 groups of stone cells are found scattered throughout the pulp 

 of such fruits as the pear and quince and they give to these 

 their gritty character. 



Fig. 38. — Stone cells from different sources, i, from coffee; 2, 3 and 4, from stem of 

 clove; s and 6, from tea leaf; 7, 8 and 9, from powdered star-anise seed. (After Moeller.) 



The uses of stone cells are apparent. In nuts and stone fruits 

 they box up, against injury and loss, the embryo and other 

 tender parts of the seed, and are in such cases of the nature 

 of an exoskeleton such as a turtle has among animals. In barks, 



