78 PLANT LIFE. 



secretions of some kind, (3) by mimicry, (4) by spiny 

 projections. 



The Date, as has been already stated, is protected by 

 the stony character of its own food, and the seed-coat 

 proper is of a very thin, papery character. Generally 

 speaking, it is the seed-coat itself which becomes either 

 stony or very tough. Sometimes the cells which 

 compose it are" very well worth examination under the 

 microscope. They have a shape like the keystone of a 

 bridge and are exceedingly thick-walled. 



But any part of the fruit or seed may become hard and 

 dry (see p. 41); the " stone " of the plum and cherry 

 is the " endocaj'p " or lining of the carpel, and so also is 

 the " parchment " of the Coffee berry. In the Hazel nut 

 and Acorn, and in the small dry fruits of the Buttercup, 

 the entire carpel becomes hard and stony. It is worth 

 noting that the development of the teeth of the squirrel, 

 to take an example of a seed-eating animal, and that of 

 the hardness of the shell, seem to have progressed side 

 by side. A squirrel does not crack the shell without a 

 little trouble, and no doubt the nut is growing thicker 

 and the squirrel's jaws stronger every year. One of the 

 most extraordinary fruits is the Brazil nut. The seeds 

 are each surrounded by a stony seed-coat and form the 

 ordinary " nuts," but the fruit which contains these 

 seeds is dry and has a very hard woody shell, so that 

 the food material is doubly protected. 



It is possible to measure degrees of hardness by 

 comparison with standard substances, such as quartz, 

 flint, calcite, etc. This is done by observing whether 

 the surface can be scratched by one or other of these 

 bodies of standard hardness. By experiment on these 

 lines, it has been shown (Ott) that the seeds of Celtis 

 and the shell of Pinus Pinea are as hard as Calcite^ the 



