REPRODUCTION. 251 



We also find that biennial plants of our climate become annual in 

 warmer climates ; perennial plants of warmer climates sometimes be- 

 come annual in our climate {Ricinus). 



4, Tree-like plants sometimes reach an old age. Of our indige- 

 nous trees the linden, oak, pine, and yew may become 1000 years old, 

 the yew even 3000 years. Among conifers the ages of Taxodium 

 distichum (Mexico) and Wellingtonia gigantea (California) have 

 been estimated at 4000 years. Of monocotyledons, Dracaena 

 Draco (Teneriffe) reaches the age of several thousand years. The 

 climax of mass development and age is reached in Adansonia digitata 

 (Africa), which is said to live 6000 years. At the moderate height 

 of 9-12 m. this tree measures 30 m. in circumference, and has 

 branches 15-18 m. long (Seubert). 



We are in doubt as to the exact age of many subterranean 

 rhizomes and perennial plants, since we have not actually observed 

 how long a rhizome may live. 



