298 METAMORPHOSIS 



FRANK. 1892. Lehrbuch d. Botanik. Leipzig, i, 376. 



GOEBEL. 1898-1901. Organographie d. Pflanzen. Jena. 



HABERLANDT. 1896. Physiol. Pflanzenanatomie, 2nd ed. Leipzig. 



KRAUS, Gr. 1895. Annales Jardin Buitenzorg, 12, 196. 



MOLLER, A. 1895. Schimper's bot. Mitt, aus den Tropen, Jena, 7, 1 19. 



NOLL. 1902. Sitzungsber. niederrhein. Gesell. f. Natur- u. Heilkunde. 



[NOLL. 1903. Biol. Centrbl. 23, 281.] 



PEDERSEN. 1874. Arb. d. bot. Inst. Wurzburg, i, 569. 



PFEFFER. 1887. Bot. Ztg. 45, 29. 



POPOVICI. 1900. Bot. Centrbl. 81, 35. 



RACIBORSKI. 1900. Flora, 87, 17. 



REINHARDT. 1892. Jahrb. f. wiss. Bot. 23, 479. 



RIMBACH. 1897. Fiinfstiick's Beitr. z. wiss. Bot. 2, i. 



ROTHERT. 1894. Cohn's Beitr. z. Biologic, 7, i. 



SACHS. 1872. Arb. d. bot. Inst. Wurzburg, i, 99. 



SACHS. 1873. Ibid, i, 385. 



SACHS. 1893. Flora, 77, 49. 



SONNTAG. 1887. Jahrb. f. wiss. Bot. 18, 236. 



STEBLER. 1878. Jahrb. f. wiss. Bot. n, 47. 



DE VRIES. 1880. Landw. Jahrb. 9, 37. 



WENT. 1895. Annales Jardin Buitenzorg, 12, i. 



WIESNER. 1876. Flora, 59, 467. 



LECTURE XXIV 

 EXTERNAL CAUSES OF GROWTH AND FORMATION. I 



THE form of the plant is determined by a large number of factors, which 

 we may divide into two groups ; internal factors, i. e. those originating within 

 the organism itself, and external factors, i. e. those emanating from the outer 

 world. In nature, external and internal factors always co-operate, and none 

 of the plant activities can be manifested under the influence of one set of factors 

 only. Still, for the purpose of investigation and description, we must, as far 

 as possible, keep the two series distinct. Among external factors the only ones 

 we will consider at present we may distinguish the purely chemico-physical 

 influence of the inorganic surroundings from the complex influences resulting 

 from association with other organisms. We have already dealt with the depen- 

 dence of certain functions (e.g. respiration, assimilation, &c.) on external fac- 

 tors, and have now to study the influence of these factors on growth ; we will 

 not limit ourselves, however, to this phenomenon only, but inquire into the 

 action of the outer world on life as a whole. 



It is not our intention to give a complete catalogue of all the known effects 

 produced by every individual factor ; we must content ourselves with a few 

 examples without going into every influence which affects the whole period of 

 growth, for sometimes embryonic growth, sometimes elongation or internal 

 differentiation, will be found to claim our chief attention. 



We may note in general that all external factors (PFEFFER, Phys. II, 85) 

 operate either directly or indirectly. Direct effects, where the external factor 

 provides the energy for the resulting phenomenon, are remarkably rare, while 

 indirect influences may be recognized almost everywhere. The external world 

 acts as a stimulus on the plant, and in co-operation with the special capacities 

 possessed by the plant, it brings about certain results. We may recognize 

 amongst stimuli a first series of factors which we shall term formal conditions ; 

 they are absolutely essential, if any development is to take place, they are the 

 essential conditions of vitality. Then again we have also stimuli which are non- 

 essential, but which, all the same, produce conspicuous results when applied to 

 the plant. All stimuli may be regarded first as either accelerating or retarding 



