SEEDS 



207 



and fibers which are nearly as tenacious and as resistive to 

 crushing as similar-sized pieces of steel. In mustard seed 

 the hardness is due to the outer mucilage cell layer. Fre- 

 quently the hardness is not due to the testa but to the endo- 

 sperm. The horny seeds of colchicum and nux vomica, seeds 

 whose endosperms are made up of reserve cellulose are 

 typical examples. 



FIG. 133. Ex-albuminous seed of calabar bean (Physostigma venenosum) 

 above, and below albuminous seed of nux vomica showing the dark endo- 

 sperm and the embryo with two well developed cotyledons and the hypo- 

 colyle. 



A scar forms at the point of separation of the ovule from 

 its stalk. This scar is called a hilum. The micropyle or 

 opening in the integuments of the ovule through which the 

 pollen tube enters to bring about fertilization, persists in 

 the seeds a& a small pit or depression and is known as the 

 micropyle-scar. 



Reserve Food. Seeds are classed according to their method 

 of storing reserve food as ex-albuminous and as albuminous 



