i8 GROWTH 



The secondary growth in thickness is never very rapid in the stems and roots 

 of woody plants. Reuss observed a maximal daily increase of thickness in the 

 stem of a lime of 0-21 mm. 1 F. Darwin found a maximum rate of growth in 

 thickness in the case of a cucumber fruit of o-oi mm. per minute, corresponding 

 to an increase in weight of 01 gramme 2 . 



Temporary variations in the activity of growth occur during the 

 grand period, and these may be distinctly perceptible during periods of 

 observation of a minute or so in the case of rapidly growing plants. The 

 variations usually occur at irregular intervals varying commonly from 

 two to thirty minutes, and are sometimes sudden, but may also arise 

 gradually. The first acceleration of growth is followed by a period of 

 retardation or even by a complete temporary cessation of growth. One 

 result of these variations is to cause the nutating apex to describe an 

 irregular curve in space. Similar variations in the rapidity of growth are 

 shown by slowly growing plants, provided that the observations are 

 sufficiently prolonged. 



These oscillations are autonomic in character, since they continue 

 under constant external conditions. Similar rhythmic variations are shown 

 by cilia, by pulsating vacuoles, and by streaming protoplasm when 

 circulation turns into rotation and subsequently reappears. Hence it is 

 hardly surprising that growth should also show variations of internal origin. 



Sachs was correct in considering these growth-variations to be autonomic 

 changes, even although the true undisturbed autonomic variations were never 

 obtained either in his researches or in those of Reinke, Drude, and others 3 . For during 

 the experiments the external conditions were not kept perfectly constant, and 

 sufficient attention was not paid to the fact that the mere attachment of a thread, 

 or the friction against the cover-glass in the case of microscopic objects, is sufficient 

 to cause disturbances of growth. As a matter of fact, however, even when all 

 these errors are avoided, autonomic variations of growth are shown by fungal 

 hyphae in air and in water, and also by roots. Moreover two similar objects 

 placed side by side may exhibit different variation rhythms. 



The disturbing influences exercised in vascular plants by the tissue-strains, 

 and by the mechanical retarding action of enclosing leaf-sheaths, are absent 



p. 247. On the leaves ofMusa see Benecke, Ber. d. Bot. Ges., 1893, p. 473 ; Maxwell, Bot. Centralbl., 

 1896, Bd. LXVII, p. i. On the leaves of Ndumbium cf. Miyake, Bot. Magazine, Tokio, 1891, 

 No. 141. Summaries are given by Pfitzer, 1. c., 1882. 



1 H. Reuss, quoted by Biisgen, Bau u. Leben d. Waldbaume, 1897, p. 65. 



2 F. Darwin, Annals of Botany, 1893, Vol. xxvill, p. 485; cf. also Kraus, Sitzungsb. d. naturf. 

 Ges. zu Halle, 1880, p. 94. 



3 Sachs, Arb. d. Bot. Inst. in Wiirzburg, 1872, Bd. I, p. 103; Reinke, Unters. ii. d. Wacbsthums- 

 geschwiadigkeit, 1872 (Verhandl. d. Bot. Ver. f. Brandenburg, Bd. xiv) ; Bot. Ztg., 1876, p. 122 ; 

 Drude, 1. c., 1881, Bd. XLIII, p. 247 (Phanerogams) ; Hofmeister, Jahresb. des Ver. filr Naturkunde in 

 \Viirttemberg, 1874, Bd. XXX, p. 222 (Spirogyra), Ffitzer, Monatsb. d. Berl. Akad., 1872, p. 384 

 (Ancylistes); Errera, Bot. Ztg., 1884, p. 497; Reiuhardt, Jahrb. f. wiss. Bot, 1892, Bd. XXIII, 

 p. 479 (Fungi). 



