PARTS OF SEEDS: THE BEAN 



Fig. 5- 



Bean seeds soaking and swelling; read 

 from left to right. 



swells by absorption of water and the seed becomes plump 



again. When the seed is split in two lengthwise by cutting the 



seed coats along the convex side, the two halves can be laid open. 



These two fleshy bodies are the seed leaves or cotyledons. The 



root or radicle lies at one end next the seed coats (see fig. 6) and 



in the entire bean seed causes 



the short elevated line near this 



end of the seed. At the opposite 



end of the radicle is the plumule, 



with its two membranous leaves 



now somewhat triangular in 



form and marked with fine 



lines or veins. The stem of 



the embryo is very short. It is 



that part of the embryo between the cotyledons and the radicle. 



It is called the hypocotyl, because it lies below the cotyledons. 

 10. Germination of the bean. In germination the radicle 



elongates more rapidly at first than the other parts, soon pierces 



the seed coat near the scar and 

 forms a long, slender, conical, pri- 

 mary root (fig. 7). The root hairs 

 soon appear, forming a dense velvety 

 covering over the root a little distance 

 back from the tip. As the stem, and 

 leaves of the plumule, which lie be- 

 tween the cotyledons, increase in size 

 the seed coats are ruptured by the 

 pressure and are usually cast off in 

 the soil. The part of the stem which 

 lies between the cotyledons and the 

 root now elongates very rapidly and 



arches up in the form of a loop. It is this loop which 



breaks the way through the soil, since it would be a difficult 



task for the slender stem to push the bulky cotyledons up 



ahead of it. The cotyledons are therefore pulled from the 



ground as the loop straightens up after emerging from the soil. 



Fig. 6. 



Bean seed split in two, showing 

 plantlet (radicle and plumule) at 

 one end. 



