sect, n PHYSIOLOGY 239 



organisms, such as Algae and Fungi, their whole existence may be 

 completed within a few days or even hours, and indeed some of the 

 higher herbaceous plants last only for a few weeks, while the 

 persistent shrubs and trees, on the other hand, may live for thousands 

 of years. 



After the formation of the seeds, there occurs in many plants a 

 cessation of their developmental processes, and such a complete 

 exhaustion of vitality that death ensues. Such an organic termination 

 of the period of life occurs in our annual summer plants, but also 

 takes place with plants in which the preparatory processes for the 

 formation of fruit have extended over two or more years, as in the 

 case of the 10 to 40-year-old Agave, which, after the formation of 

 its stately inflorescence, dies of exhaustion. In plants, on the other 

 hand, which in addition to the production of flowers and fruit accu- 

 mulate also a reserve of organic substance, and, with their reproductive 

 organs, form also new growing points, life does not cease with the 

 production of the seeds. Such plants possess within themselves the 

 power of unlimited life, the duration of which may only be terminated 

 by unfavourable external conditions, the ravages of parasites, injuries 

 from wind, and other causes. 



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 iu Wiirtemberg, is between 800 and 1000 

 years old ; the age of the Fir of Beque is estimated at 1200 years, and a Yew in 

 Braburn (Kent) is at least as old. A stem of a Sequoia in the British Museum 

 has with 1330 annual rings, a diameter of 4 - 5 m., and, according to Cakkuthers, 

 must have originally been 28 '5 m. in circumference. An Adansonia at Cape Verde, 

 whose stem is 8 - 9 m. in diameter, and a Water Cypress, 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 

 aee ■ 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 hundred years. 



In thus referring to the ages of these giant plants, it must not 

 be understood that all the cells remain living for so long a time, 

 but rather that new organs and tissues are developed, which continue 

 the life of the whole organism. All that is actually visible of a 

 thousand-year-old Oak is at most but a few years old. The older 

 carts are dead, and are either concealed within the tree, as the pith 

 and wood, or have been discarded like the primary cortex. The cells 

 of the original growing point have alone remained the whole time 

 alive. They continue their growth and cell division so long as the tree 

 exists • while the cells of the fundamental tissue arising from them, 



