SPECIFIC GRAVITY 629 



5. Weight. — The weighing of seeds is usually employed with 

 a view of determining the comparative values of samples of the 

 same species, and under certain circumstances it is an important 

 means of distinguishing the good from the bad. 



We may consider the weight of seeds from three distinct 

 points of view, viz. : (a) Weight of um'f volume of the seed as 

 compared with the weight of the same volume of water or its 

 specific gravity, (d) Weight of a definite number of seeds, as 

 100 or a 1000; sometimes spoken of as the '■absolute weight.^ 

 {c) Weight of a certain definite measure or bulk of the seeds, 

 usually that of a bushel or pint which we designate its volume- 

 weight. 



(a.) Specific Gravity. — The determination of the specific 

 gravity of seeds is chiefly of theoretical interest, and there is no 

 room for its discussion here. The method, however, of testing 

 seeds by throwing them into water and observing those which 

 sink or swim is concerned to some extent with their specific 

 gravity, so that mention of it is not out of place. 



The specific gravity of seeds depends chiefly on their chemical 

 composition and the presence or absence of airspaces inside the 

 seed, under the testa, between the cotyledons or in the endo- 

 perm tissue. As the chief constituents of seeds (e.g., starch, 

 cellulose, sugar, albuminoid material), are with the excep- 

 tion of fats and oils, heavier than water, most seeds ought to 

 sink if no air-spaces are present in them and they could be 

 readily wetted. Seeds, however, vary enormously in regard to 

 the air they contain ; moreover, to those whose surfaces are hairy, 

 rough and uneven, air-bubbles become frequently attached 

 and enable somewhat heavy seeds to float when thrown into 

 water. Some seeds which readily sink after harvesting often lose 

 water by slow evaporation when they are kept, and become 

 lighter and capable of floating owing to the penetration of air to 

 fill up the spaces left. In these cases, «.^., beans and peas, the 

 test may help us to decide which is old seed, but applied to 



