[598 



A TEXTBOOK OF THEORETICAL BOTANY 



grow and thicken, especially at its lower end. The cotyledons are com- 

 pletely united to each other, forming a tube around the plumule. Their inner 

 ends remain embedded as haustoria in the endosperm, but when this is 

 exhausted an abscission layer at the base of the tube allows the radicle to 

 drop away from them, taking the plumule with it. If the tide is low their 

 weight thrusts the seedlings upright into the mud, where they continue 



i 



o 



Fig. 1457. — Rhizophora conjugata. Branch with fruits and pendent 

 seedlings. Below, seedUngs floating and one rooted. (Frovi Ridley, 

 "The Dispersal of Plants throughout the World".) 



