SEEDS. 



SECTION V. 



POWDERED SEEDS AND FRUITS, 



The characteristic elements that are available for the identi- 

 fication of powdered seeds and fruits are undoubtedly more nu- 

 merous than those at our disposal for powdered barks and roots. 



Every seed is enveloped in seed-coats composed of several 

 layers of cells exhibiting differences in structure. In some 

 natural orders a most remarkable and constant analogy is to be 

 found in the structure of the seeds, as for instance in 

 Leguminosse, but when the seeds of one natural order are com- 

 pared with those of another, very striking and characteristic 

 differences may be detected. These differences are to be found 

 chiefly in the number of layers of which the seed-coats are com- 

 posed, their arrangement and structure, which is often very 

 complex, as well as their colour, the presence or absence of endo- 

 sperm and the nature of the substances contained in both endo- 

 sperm and embryo. It may, however, here be observed that the 

 mostexact and most constant, consequently, therefore, the most 

 important of all the diagnostic characters, is usually to be found 

 in the sclerenchymatous layer. Apart from their particular 

 colour, which is the first thing that attracts the attention of the 

 microscopist, the cells of which this layer is composed exhibit a 

 remarkable constancy of shape in the same seed, and striking 

 differences from the corresponding cells in the seeds of other 

 species. To such an extent is this the case that these cells often 

 furnish a means not only of identifying a single seed, but of dis- 

 tinguishing all the seeds belonging to a particular natural order 

 or genus, 



