546 PHYSIOLOGY. 



just been noticed that annuals may be made to live several years by pre- 

 venting them from flowering. The arrest of growth of the large and 

 highly developed axes of monocarpic perennials (Agave, Talipot Palm, 

 &c.) is a necessary consequence of the terminal bud producing blossom 

 instead of leaves ; but the formation of propagative offsets from the leaf- 

 axils before death is strictly dependent on the degree of vigour possessed 

 by the main axis at the time of flowering. 



The duration of herbaceous perennials may be regarded as unlimited, 

 since they are always placed in a position to form new absorbing organs 

 (roots) in the vicinity of their buds. The duration of trees is also 

 theoretically unlimited ; and in many cases great age is attained; but 

 ordinarily trees acquire increased vigour with age up to a certain point, 

 and then begin to decline, a circumstance attributable to the increasing 

 distance to which the buds are removed from the roots, the obstruction 

 to the flow of sap, the local decay of the roots and trunk from external 

 injuries, &c. Cuttings from old trees, if taken from sound shoots, may 

 be made the foundation of new trees as vigorous as the parents were in 

 their earlier years. 



Palm-trees grow to an age of 200 years or more ; the Draccena (Dragon- 

 trees) of Teneriffe have been known as old trees for centuries. Oaks, 

 Limes, Cedars, Yews, &c. are known to have lived many centuries; and 

 other cases are on record of gigantic trees whose age, deduced from the 

 number of rings of growth of the stems, would amount to upwards of 

 3000 years. The Bertholletice of Brazil, the Adansonice of Senegal, and 

 the Wellingtonia or Sequoia gigantea of California (363 feet high and 31 

 feet in diameter at the base) are examples of this. 



Death of the Plant. In herbaceous perennials the older parts 

 of the plant die and decay in a limited period after the develop- 

 ment of the new axes. In Dicotyledonous trees also the older 

 part, which is enclosed by the new layers, and becomes consolidated 

 into heart-wood, must be regarded as dead after a certain period, 

 ceasing even to carry sap mechanically ; and we see hollow trees 

 of this Class living and growing, where the whole of the older 

 part has been lost by decay, a living shell of wood constituting the 

 bond of connexion between the roots and the growing branches of 

 the axis. 



This death of the older tissue is not so common in arborescent 

 Monocotyledons ; but it is observed in Pandanus (fig. 10), where the 

 base of the stem and the old roots decay, new (adventitious) roots 

 sprouting out from the living part of the trunk in a continual ad- 

 vance upwards. 



The death of a plant or part of a plant depends upon the death of the 

 cells composing its tissues. The duration of the life of individual cells is 

 very different, according to their position and function. Cells situated at 

 growing-points (in buds, cambium-regions, tips of roots, &c.) are very 

 transitory, since during active vegetation they are continually divided, as 

 parent cells, into two or more new cells, part of which are left behind as 

 permanent cells, those situated at the periphery, or most advanced point, 



