274 BOTANY tart i 



a maximum is attained. "With the same natural necessity the ascend- 

 ing branch of the curve proceeds until finally terminated by the death 

 of the tree after the latter has for many years produced seeds to 

 continue the species. Long before the whole individual perishes, 

 however, single parts of it have died. Thus, the leaves have been 

 shed after persisting for one or several years. In some cases whole 

 branches are shed, though often they perish without being thrown oft' 

 and gradually break up while still attached to the plant. All the older 

 tissues of the stem also die ; the peripheral tissues are transformed 

 into bark and either fall oft" or form a protective covering to the parts 

 within. In the centre the wood is transformed into heart-wood in 

 which the remaining living elements die. In an old tree only the 

 growing points, whether apical or intercalary, and the youngest tissues 

 derived from them remain alive. Thus we see that every cell Avhich 

 has lost its embryonic character dies after a longer or shorter time, 

 if it does not under special conditions become again embryonic. 



The duration of life of a tree though in principle unlimited is determined by 

 external conditions. An end comes to its existence owing to the attacks of parasites 

 and other enemies, the effect of wind, or especially to disturbances in its nutrition. 



The longevity of trees having an historical interest is naturally best known and 

 most celebrated, although, no doubt, the age of many other trees, still living, dates 

 back far beyond historical times. The celebrated Lime of Neustadt in Wurtemberg 

 is nearly 700 years old. Another Lime 25"7 m. in circumference had 815 annual 

 rings, and the age of a Yew in Braburn (Kent) which is 18 ra. in circumference is 

 estimated at 2880 years. A stem of a Sequoia in the Berlin museum has, with 

 1360 annual rings, a diameter of 4 '7 m., from which an idea can be formed of the 

 age of those trees which have attained a diameter of 16 m. An Adansonia at 

 Cape Verde, whose stem is 8-9 m. in diameter, and a ^Vater Cypress {Taxodium 

 iiicxicanum) near Oaxaca, Mexico, are also well-known examples of old trees. Of 

 an equally astonishing age must have been the celebrated Dragon tree of Orotava, 

 which was overturned in a storm in 1868, and afterwards destroyed by fire. The 

 lower plants also may attain a great age ; the apically growing mosses of the 

 calcified Gymnostomum clumps, and the stems of the Sphagnaceae, metre-deep in 

 a peat-bog, must certainly continue to live for many centuries (™). 



V. Reproduction 



Some trees attain, as we have seen, a great age and are in principle 

 capable of unlimited life. After a certain maximum in their develop- 

 ment has been reached, however, the new shoots formed annually 

 become smaller, obviously because the exchange of material between 

 the roots and shoots becomes increasingly difficult. Ultimately the 

 tree dies owing to this or other causes. If new roots can be induced 

 to form near to the growing points the intensity of the growth of 

 the shoots is maintained and under these conditions no limit can 

 be placed to tlie growth. This experiment cannot be carried out in 

 the case of every tree since all do not form roots readily ; it is, how- 



