GROWTH UNDER CONSTANT EXTERNAL CONDITIONS 9 



usually does not exceed 10 mm. long in terrestrial and aquatic roots, and 

 may not be more than 2 to 3 mm. in length in very small roots, or 

 even less in other small objects. The latter form a transition to those 

 fungi in which growth in length is restricted to the extreme apex of 

 the elongating hypha, so that the growing zone may be less than o-oi mm. 

 long, and no distinction between embryonic and growing zones is possible. 



Very small objects may be dusted with starch, red lead \ &c., or accidentally 

 adhering particles may be used as marks, as may also local thickenings, pits, &c. 2 

 The length of the growing region can also be estimated from the relative length 

 of the internodes at the growing apex, or of the cells in the case of 

 an algal filament. Thus in Fig. 3, if the cell 4 is equal in length 

 to 5, it has ceased to grow, and if when 3 becomes equal to 4, 

 a new segment 2 has appeared, it is evident that the zone of growth 

 extends as far as the third segment. The method is not however 

 very accurate, since all the cells and internodes do not attain the 

 same adult length 3 . 



At the apex of the stem the products of division develop 

 differently, according to whether they form part of a nodal 

 or internodal zone, for it is only the latter region that under- 

 goes pronounced growth in length when a bud unfolds. The 

 same applies to Nitella, in which alternate segments from 

 the apical cell form the nodes and internodes. 



Frequently also the activity of growth differs in different parts of 

 the same internode, as for example when intercalary zones of growth are 

 present at the bases of the internodes, as in the stems of grasses, docks, 

 Equisetums, Cannas, &c. These zones have only a limited power of 

 growth, which may suffice to add as much as 80 mm. to the length of 

 an internode of Polygonum orientate and Canna indica, or even more 

 than this in Molinia caerulea*. Many leaves (Allium, Tulipa, Welwitschid) 



1 Haberlandt, Function u. Laged. Zellkerns, 1887, p. 55 ; Reinhardt, Jahrb. f. wiss. Bot., 1893, 

 Bd. xxin, p. 552 ; Pfeffer, Unt. a. d. Bot. Inst. zu Tubingen, 1886, Bd. II, p. 277, footnote; Ewart, 

 Trans. Liverpool Biol. Soc., Vol. vin, 1894, p. 243. [Platinum black or the precipitated oxides of 

 manganese are much more suitable.] 



2 For examples see Nageli, Pflanzenphysiol. Unters., 1855, Bd. I, p. 60 ; Nageli and Schwendener, 

 Mikroskop, 1877, 2. Aufl., p. 545 ; Noll, Unters. iiber das Wachsthum d. Zellmembran, 1887, p. 129; 

 A. Nathansohn, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 671. 



3 Details are given by Askenasy, 1. c., 1878, Bd. II, p. I seq. 



4 Grisebach, Archiv f. Naturgesch. von Erichson, 1843, Bd. IX, I, p. 275, and 1844, Bd. X, I, 

 p. 134. Also Hartig, Linnaea, 1847, Bd. xix, p. 479 ; Munter, Linnaea, 1841, Bd. xv, p. 209, and 

 Bot. Ztg., 1843, p. 69 ; Sachs, Arbeit, d. Wiirzb. Inst., 1872, Bd. I, p. 127, and Flora, 1873, p. 323 ; 

 Strehl, Unters. iiber Langenwachsthum d. Wurzel u. des hypocotyl. Gliedes, 1874; Bennet, Bot. 

 Jahresb., 1876, p. 743; Askenasy, I.e., 1878; Riitzow, Bot. Centralbl., 1882, Bd. ix, p. 82; 

 Wiesner, Sitzungsb. d. Wien. Akad., 1883, Bd. xcvin, Abth. i, p. 454; Schwendener and Krabbe, 

 Jahrb. f. wiss. Bot., 1893, Bd. xxv, p. 340; Rothert, Cohn's Beitr. z. Biologic, 1896, Bd. vn, p. 77. 

 Summaries are given by Hofmeister, Allgem. Morphol., 1868, pp. 417, 528. Cf. also Goebel, Vergl. 

 Entwickelungsgeschichte der Pflanzenorgane, 1883. 



