GROWTH UNDER CONSTANT EXTERNAL CONDITIONS 5 



No such direct repetition is possible in the case of those somatic 

 cells which undergo permanent tissue differentiation, and hence whose 

 development terminates with the assumption of their adult form. During 

 this development very great growth in size may take place, as for example 

 when a young internode of Phaseolus or Humulus elongates more than 

 a hundred times, or when a tiny leaf-rudiment develops into a large adult 

 leaf. At first this increase in size is usually accompanied by cell- 

 division, but the latter gradually decreases, so that usually the most active 

 growth is mainly or entirely due to the increase in size of the individual cells. 



The most rapid growth in Somatophytes usually takes place a little 

 behind the growing apex, and hence a comparatively small amount of 

 cell-division in the primary meristem is sufficient to give rise to a con- 

 siderable total growth. All plants do not however follow this rule, which 

 indeed does not apply at all to Asomatophytes. For example, in 

 the case of bacteria, remarkably active growth in size takes place in 

 embryonic cells, while in the rapidly growing hyphae of Botrytis, Mucor, and 

 AspergilliiS) the growing zone is restricted to the extreme apex, and is 

 frequently not longer than the diameter of the hypha *. Similar relation- 

 ships hold good for such algae as Vaucheria and Caulerpa^^ for rhizoids 

 and moss protonemas, and even for the root-hairs of Phanerogams 3 . 

 Hence Sachs' supposition that the most active growth could never occur 

 at the embryonic apex is obviously incorrect. Indeed Westermaier found 

 that in the growing apices of Vascular Cryptogams the most active increase 

 in size takes place in or near to the apical cell 4 . 



In the case of the unicellular Somatophytes Vaucheria and Mucor, 

 the entire plant retains the embryonic condition, but the growth of the 

 cell-wall, and hence the elongation of the filaments or hyphae, takes place 

 at their apices only. The same applies to Aspergillus, Penicillii.m, and 

 Sphacelaria 5 , in which plants transverse divisions appear a little distance 

 behind the growing apex. On the other hand, the lateral outgrowths of 

 Characeae, Pterothamnion^ and Callitkamnion 6 are formed from segments of 



1 On Botrytis see Reinhardt, Jahrb. f. wiss. Bot., 1892, Bd. XXill, pp. 494, 554, and the literature 

 there quoted; also Eidam, Cohn's Beitrage, 1887, Bd. IV, p. 209; Mucor\ Errera, Bot. Ztg., 1884, 



PP- 535, 564- 



2 Askenasy, Neue Methode, d. Vertheilung d. Wachsthumsintensitat zu bestimmen, 1878, p. 28 

 (Verhandlgn. d. naturw.-med. Vereins zu Heidelberg, N. F., Bd. II, Heft 2) ; Berthold, Protoplasma- 

 mechanik, 1886, p. 274; Reinke, Ueber Caulerpa, 1899, p. 71 (Wissenschaftl. Meeresunters., Kiel, 

 N. F., Bd. v). 



3 Haberlandt, Function u. Lage des Zellkerns, 1887, and Oesterr. Bot. Zeitschrift, 1889, No. 3; 

 Reinhardt, 1. c., p. 552 ; Sokolowa, Wachsthum der Wurzelhaare u. Rhizoiden, 1897. 



* Sachs, Vorlesungen, 1887, 2. Aufl., p. 558; Westermaier, Jahrb. f. wiss. Bot., 1879-1881, 

 Bd. xn, p. 439- 



5 Geyler, Jahrb. , wiss. Bot., 1865-1866, Bd. IV, p. 479 ; Pringsheim, Ueber d. Gangd. morphol. 

 Differencirung d. Sphacelarien-Reihe, 1873, p. 145. 



' Nageli, Pflanzenphysiol. Unters., 1855, Bd. I, p. 60; Askenasy, 1. c., p. 28. 



