PHYSIOLOGY 263 



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 Wurtem- 

 berg is nearly 700 years old. Another Lime 257 in. in circumference had 815 

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

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

 1360 annual rings, a diameter of 47 m., from which an idea can te 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 Water Cypress ( Taxodium 

 mexieantiM) 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 

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

 a peat-bog, must certainly continue to live for many centuries ( 74 ). 



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 

 parts 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 somatic cells of the permanent tissue arising 

 from them, and destined for particular purposes, all lose their vitality 

 after a longer or shorter performance of their functions. 



The cells of the root-hairs often live for only a few days ; the same is also true 

 of the glandular cells and trichomes of stems and leaves. The wood and bast fibres, 

 as also the sclerenchymatous cells, lose their living protoplasiu after a short time. 

 Entire organs of long-lived plants have frequently but a short existence ; the sepals, 

 petals, and stamens, for example. The foliage leaves, also, of deciduous trees live 

 only a few summer months and then are discarded. The leaves even of evergreen 

 plants continue living but a few years, ' before they too fall off. Small twigs, 

 especially of Conifers, are also subject to the same fate. The cells of the medullary 

 rays afford the best examples of long-lived cells constituting permanent tissues. 

 In many trees, as in the Beech, living medullary ray cells over a hundred years 

 old have been found, although, for the most part, they live only about fifty years. 



V. The Phenomena of Movement 



In every living organism there is constantly occurring in the 

 course of the metabolic processes an active movement and transposi- 

 tion of substance. As these movements are for the most part 

 molecular they are generally imperceptible ; but that they actually 

 take place is demonstrated with absolute certainty by the local 

 accumulation and diminution of substance. 



There are also other forms of movement which play an important 



