DIV. ii PHYSIOLOGY 309 



D. Duration of Life 



We have further to consider the periodic alternation expressed in 

 the duration of life of the plant as a whole. There are plants, such 

 as Stellaria media and Senecio tulgaris, which in a few weeks go through 

 their whole development from the germination of the seed to the ripen- 

 ing of their seeds. Since each seed can germinate at once, several 

 generations may be developed within the year. The individual plant 

 dies on producing a certain number of seeds, but the seeds ensure the 

 maintenance of the type of plant. Many annual plants are similar 

 though their life is more closely connected with the seasons of the 

 year. With these may be placed other plants which only fruit once 

 (monocarpic) but in which seed-formation is preceded by two or 

 many years of purely vegetative growth, with or without resting periods. 

 Probably in all these cases the development of fruit is the cause of 

 the death of the vegetative organs, for their life can be considerably 

 prolonged by preventing seed-formation. In contrast to these plants, 

 others, such as our native trees, fruit repeatedly, the existence of 

 the individual not being terminated by seed-formation. All perennial 

 types exhibit another periodicity besides that due to the seasons. 

 A tree in its first year when it is a seedling has less intensity of 

 growth than many annual plants ; the intensity of growth increases 

 gradually and its growth in length, its growth in thickness, and even 

 the elementary organs of the wood continue to increase in size until 

 a maximum is attained. Some trees attain a great age and are 

 capable of unlimited growth. From a certain point of maximum 

 development, however, the annual shoots become smaller, apparently 

 on account of the increased difficulty of exchange of materials between 

 the roots and leaves. Ultimately the tree dies for this reason, or 

 owing to the attacks of parasites or other disturbing external effects. 

 If care is taken to ensure the production of new roots near the 

 growing points of shoots, the latter will continue to grow \vith 

 the same intensity, and no termination of the growth is to be 

 anticipated. This experiment cannot be performed on every tree, 

 since some do not readily give rise to roots ; it is easily done 

 with the Willow, however, by using branches as cuttings. 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 off 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 off or form a protective covering to the parts within. In the 

 centre the wood is transformed into heart-wood in which the remain- 

 ing living elements die. In an old tree only the growing points, 

 whether apical or intercalary, and the youngest tissues derived 



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