350 METAMORPHOSIS 



This hypothesis very conveniently explains anomalies and regeneration 

 phenomena, and this has won it a certain amount of acceptance. Closer examina- 

 tion shows, however, that the difficulties are not thereby removed, but only 

 shifted elsewhere. If we ask what these specific constructive bodies are, why 

 they pass to certain definite places, and how their morphogenetic activity is con- 

 ditioned, we are compelled to say we have no knowledge ; hence it has been said 

 that SACHS'shypothesis is merelya paraphrase of the facts, which moreover is not 

 correct. A criticism of this hypothesis is to be found in Vochting (1899), 

 Pfeffer (Phys. II, 234), and Klebs (1903) [also Goebel, 1905]. According to 

 our view, the specific constructive substances for each organ cannot be trans- 

 ported from place to place, they consist of protoplasm,\vh.\(^ remains stationary. 

 We must assume, however, that the protoplasm of every young cell has the power 

 of constructing the most varied type of organ. That it forms one organ rather 

 than another is determined by a releasing force unknown to us and not by any 

 primary constructive material. 



Side by side with the external ' metamorphosis ' in the shoot is another 

 ' metamorphosis ' in stems, branches and roots, whose seat is not the apical 

 meristem but the intercalary meristem known as cambium. As is well known, 

 this cambium produces different tissues inwardly and outwardly, which are 

 termed secondary wood and secondary bast. The construction of the wood 

 takes place in such a way that, in the course of the vegetative period, the 

 cambium does not always produce similar elements, those formed in the begin- 

 ning of spring are different from those formed in the height of summer, and 

 these we term spring wood and autumn wood respectively (for literature see 

 Wieler, 1891, 1892, 1897 ; JosT, 1891, 1893) Spring wood merges gradually 

 into autumn wood but the transition from autumn wood to spring wood is more 

 abrupt, and hence the various annual rings or single year's growths are often 

 clearly visible to the naked eye. The difference between spring and autumn 

 wood in the simplest cases, e.g. the Coniferae, lies merely in the decrease in the 

 diameter of the tracheides and the increase in the thickness of their walls. 

 This simple condition had long since suggested an explanation of annual rings. 

 Sometimes purely mechanical factors, such as the pressure of the cortex, some- 

 times nutritive influences were given as the causes. No one believes any longer 

 in any real influence of cortical pressure on annual ring formation, but it is un- 

 deniable that cells may develop very differently under different nutritive con- 

 ditions ; so long, however, as we have no experimental data available as to the 

 influence of nutritive substances on simple objects any explanation of secondary 

 increase in thickness must remain imperfect. 



These explanations are, besides, quite inadequate when we thinkof the annual 

 rings of Dicotyledons, where, in addition to quantitative differences, qualitative 

 differences also occur between autumn and spring wood, as, for example, in the 

 more abundant or exclusive formation of large vessels in spring. It is naturally 

 impossible to give a definite answer as to the effect of nutrition in such specific 

 cases, and so no theory has been hazarded as to the special problem of annual 

 ring formation. We must simply accept the view that this periodicity in the 

 annual ring is due to internal factors, just as we have regarded the yearly 

 periodicity in longitudinal growth also as autonomous. But just as in opposition 

 to the rule there are conditions when a second shoot in the same year may be 

 observed, or induced to form experimentally, so it is, too, with secondary 

 growth ; a second annual ring may be seen in the horse-chestnut in autumn 

 when supplementary shoots appear. A certain relation, in fact, subsists 

 between the annual ring and the annual shoot. If the second shoot appears 

 before the autumn wood has been formed, one would not expect a second 

 annual ring (oak, Lammas-shoot) ; the relations between annual rings and 

 annual shoots do not, however, appear to be of the nature of correlations as 

 was previously believed (Jost, 1891), so that the formation of leaves may not 



