PERIODICITY IN DEVELOPMENT. I 



351 



be the direct consequence of spring wood formation, both phenomena may 

 depend on common factors, factors which operate in such a way as to induce 

 the formation, after a certain resting period, of a new shoot with fohage-leaves 

 and a new zone of wood with broad vessels. This concurrence is always 

 appropriate, since with every new leaf a rise of transpiration takes place, to 

 replace which increase in the water conduits is necessary. As to the annual 

 rings in tropical trees so little is as yet known that it is needless for us to consider 

 them (HoLTERMANN, 1902 ; [Ursprung, 1904]). 



In addition to the daily and yearly periodicity in the plant a few words 

 may be said in conclusion as to periodicity in its entire life-cycle. There are 

 plants like Stellaria media, Senecio vulgaris, &c., which go through their entire 

 life-history, from the germination of the seed to the ripening of new seed, in 

 a few months, in which also each seed germinates at once, so that several genera- 

 tions may be formed in one calendar year without any rest and unrestricted by 

 the time of year. After a certain number of seeds have been formed the plant 

 dies off, but the seeds provide for the maintenance of the species. Similarly, but 

 more intimately associated with the seasons are numerous annuals, and with 

 these forms may be associated other monocarpic or single fruiting plants, in 

 which the formation of seed is preceded by a two to many year stage of purely 

 vegetable growth with or without interpolated resting periods. The proba- 

 bility is that in all these cases the formation of fruit is the cause of the death of 

 the vegetative organs, since life in the latter may be markedly prolonged by 

 preventing the setting of seed. To a certain extent in opposition to these types 

 there are plants, such as our native trees, which fruit frequently, and whose 

 continued existence is not determined by the formation of seed. In all perennial 

 types another periodicity, in addition to the annual periodicity, makes its ap- 

 pearance, which we will study only as it is represented by trees. A tree is at the 

 commencement a seedling with very limited power of growth, as is seen in many 

 annuals; it gradually gains m strength, however, and by growth in length and in 

 thickness, and by the development of the constituent elements of the xylem 

 it attains ever increasing size until a maximum is reached. Similarly there 

 arises also as a natural necessity a descending curve, which finally ends in death 

 after the tree has gone on for many years producing seeds for the maintenance 

 of the species. Long before the specimen in question as a whole succumbs, 

 individual parts of it die off. Thus the leaves die off after they have performed 

 their functions for one or more years, and although external factors co-operate 

 in the leaf-fall of deciduous trees, still \ea.i-fall is as much an organic process 

 as \e.dA-jormation. As a rule, certain cells are produced at the base of the leaf- 

 stalk whose function it is to cut the leaf off. These cells form a separating layer, 

 the swelling up of a certain middle lamella of which brings about the separation 

 of the dying part from that which still remains alive. As in the case of leaves, so 

 also entire branches may be abstricted, or, without any such separation, may die 

 and gradually rot away where they were developed. All the older tissues of the 

 stem die in the long run ; the peripheral tissues become transformed into bark, 

 falling off or forming a protective sheath to the parts within ; centrally the wood 

 becomes transformed into duramen involving death of the elements. Only the 

 apical and intercalary merismatic regions, as also their youngest derivatives, re- 

 main alive in an old tree. We thus see that every cell which has lost its embryonic 

 characters dies after a longer or shorter period if it does not assume these 

 characters anew, for reasons of which we have spoken in Lect. XXVI. Whether, 

 however, a cell remains or becomes embryonic depends on its relations to the 

 whole plant and to its different parts, for the organism provides for the per- 

 sistenceof some cellsand thedeath of others. But this is not true of all organisms; 

 where there isnodifferentiation into embryonic and somatic cells, as in unicellular 

 types, there are no cells to die off of their own accord, all remain alive so long as 

 they are not injured by accidents from without. We will refer again to ihis 



