326 METAMORPHOSIS 



Magnus, W. 1903. Bcr. d. bot. Gesell. 21, 129, 



MoLLER. 1893. Flora, 77, 254. 



MOTTIER. 189Q. Annals of Botany, 13, 346. 



Nabokich. 1901. Ber. d. bot. Gesell. iq, 222, 



Nabokich. 1902. Beihefte Bot. Centrbl. 13, 272. 



[NiKiTiNSKV. 1904- Jahrb. f. \viss. Bot. 40, i.] 



Noll. 1900. Landw. Jahrbiicher, 29, 361. 



Noll. 1901. Sitzungsber. niederrh. Gesell. 



Oltmanns. 1 89 1. Sitzungsber. Berliner Akad. d. Wiss. 



[Perodko. 1904 Jahrb. f. wiss. Bot. 41, i.] 



Peyritsch. 1882. Jahrb. f. wiss. Bot. 13, i. 



Pfeffer. 1893. Druck u. Arbeitsleistung. (Abh. Kgl. Gesell. Leipzig, 20, 233.) 



PuLST. 1902. Jahrb. f. wiss. Bot. ^,7^ 205. 



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



RicHTER. 1894. Flora, 78, 423. 



ScHiMPER. 1 89 1. Indo-malayische Strandflora. Jena. 



ScHWARZ, F. 1 88 1. Unters. Bot. Inst. Tiibingen, i, 53. 



ScHWARZ, F. 1883. Ibid. I, 135. 



Stahl. 1894. Bot. Ztg. 52, 117. 



Tucker and Seelhorst. 1898. Journal f. Landw. Reviewed in Biedermann's 



Jahresber. 28, 269. 

 VocHTiNG. 1878. Die Organbildung, i. Bonn. 

 [Wiedersheim. 1902. Jahrb. f. wiss. Bot. 2,^, 41.] 

 WiELER. 1883. Unters. aus d. bot. Inst. Tubingen, i, 189. 

 WiELER. 1 90 1. Ber. d. bot. Gesell. 19, 366. 

 WiESNER. 1 89 1. Ber. d. bot. Gesell. 9, 46. 

 WiESNER. 1895. Ibid. 13,481. 

 WiESNER. 1896. Ibid. 14, 180, 



WORTMANN. 1 88 1. Bot. Ztg. 39, 368. 



LECTURE XXVI 



CORRELATIONS 



Since, as we pointed out at the end of the last lecture, an alga living sym- 

 biotically with a fungus can markedly influence the mode of growth of the other 

 member of the partnership, although a give-and-take of soluble materials is 

 almost the only relation that subsists between them, and, further, since an 

 insect, which presumably operates only by excreting some chemical substance, 

 can induce the formation of galls, and, though a foreign organism, bring about 

 far-reaching modifications in plant form, it follows that individual organs, bound 

 together by intercellular protoplasmic threads into one organic unity, must 

 also influence each other in a very marked manner. Such relationships of 

 plant-organs, which may be termed 'growth-correlations ' (Goebel, 1880), are 

 very generally met with and claim our attention here. They stand on the 

 border-line between internal and external growth-factors, for if we regard the 

 individual cell or growing point as relatively independent, influences exerted on 

 it by other cells or other growing points of the same plant may be regarded as 

 external influences, since all parts of the plant, save that immediately under 

 consideration, are in the position of an environment to it. If, however, we look 

 upon the entire plant as a single unit then the action of one part on another 

 must be considered as that of an internal factor. 



The cells of Spirogyra, which are all alike and which all perform the same 

 functions, do not apparently influence each other's growth, and it would appear 

 that, so far as the welfare of the individual cell is concerned, it is immaterial 

 whether they be united to each other or not. 



If, however, different cells or, speaking more generally, different organs of 

 a plant-body differ in structure and function, then they necessarily influence 



