240 BOTANY 



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 sclerenehymatous cells, lose their living protoplasm 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, after being partly emptied of their 

 contents, are discarded. 



Before the falling of leaves a separative layer is first formed in the elongating 

 leaf-base (p. 143) ; while a layer of cork, formed either before or after the leaf- 

 fall, closes the leaf-scar. The formation of ice in the absciss layer, as may easily 

 be observed after the first frost, facilitates the separation of the leaf from the 

 stem. 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 con- 

 stituting 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. 



Continuity of the Embryonic Substance. — While the cells of the 

 permanent tissue have thus but a limited activity, the vitality of 

 the embryonal tissues is unlimited, and never terminates from natural 

 causes. From such embryonal tissue the growing points of perennial 

 plants are formed, and the growing points of their descendants, as 

 Sachs has pointed out, are also derived from it, through the substance 

 of the sexual cells. The embryonic substance does not change ; it 

 produces new individuals, which live but a short time, but is itself 

 perpetuated in their offspring ; it continues always productive, always 

 rejuvenescent and regenerative. The thousands and thousands of 

 generations which have arisen during the past ages were its products ; 

 it continues living in the youngest generations with a capacity for 

 production still unabated and undiminished. The single organism is 

 perishable ; its embryonic substance, however, is imperishable and 

 unchangeable, and continually gives rise to new tissues. Considered 

 from this standpoint, the difference between short- and long-lived 

 plants, between annual herbs and thousand-year-old trees, appears in 

 quite another aspect. From the embryonic substance of the oldest 

 trees there are produced, each year, new leaves and shoots, which, 

 however, remain united with the dead remains of former years. In 

 annual herbs, on the other hand, the embryonic substance in the 

 embryo becomes separated each year from the dead plant, and develop- 

 ing new leaves, stems, and roots, forms a completely new individual. 



The old and well-known maxim of Harvey's, " Omne vivum ex 

 ovo," is, in other words, only the expression of the principle of the con- 

 tinuity of the embryonic substances. And similar to it, in its continual 



