[Chap. XXXIV GROWTH, DORMANCY, GERMINATIOiN OF SEEDS 389 



Fig. 172. Development of mangrove seedlings. The tree {Rhizophora mangle) 

 grows on soft mud flats in the tropics and semi-tropics. The seed in the fruit 

 pictured on the left germinates while the fruit is still attached to the tree. The 

 embryo grows a foot or more in length before it finally drops endwise like an 

 arrow into the mud below and becomes a seedling. Photo from Field Museum of 

 Natural History. 



Structures, and most of the food used during germination comes from the 

 enclosing endosperm. By the time the endosperm has dropped, the 

 cotyledons have doubled or trebled in area. Meanwhile they have 

 become green, and photosynthesis has begun. Further growth is de- 

 pendent on sugar and other foods made within the green seedling. 



The cotyledons of the common bean and lima bean are likewise raised 

 above the soil surface. There is no endospeiTn, and the initial growth is 

 dependent solely upon the food contained within the embryo. As the 

 growth of the plumule proceeds, the cotyledons shrivel and abscise 

 within two or three weeks. Bv this time a shoot containing two or 

 three leaves has developed from the plumule. 



The growth of roots during the germination of seeds of grasses was de- 

 scribed in Chapter XXIX. The hypocotyl of grasses does not elongate, 

 and the cotyledon remains in the soil. The plumule, which is enclosed 

 in a sheath (the coleoptile), grows upward through the soil, and by its 

 enlargement breaks through the sheath. The plumule of a grass seed is 

 thus the forerunner of all the plant that appears above the surface of the 

 soil. 



