GROWTH UNDER CONSTANT EXTERNAL CONDITIONS 7 



This law applies to animals as well as to plants, and it is not surprising 

 that the grand periods for different organs, and even for special functions 

 also, do not necessarily coincide either in amount or in time. 



The growth-curve of an internode usually exhibits a single maximum only, 

 whereas that of an entire tree shows annual summer maxima and winter minima 

 in temperature climes. In tropical regions the maxima and minima occur during the 

 wet and dry periods. The total annual growth in mass however attains a maximum 

 at a certain age 1 , before or after the first flowering and fruiting 2 . The leaves and 

 internodes 3 on every shoot increase in size from below upwards, and the length of 

 the cells increases to a maximum and then decreases again as we ascend a stem 

 or pass outwards through successive annual rings 4 . 



These differences are the result of processes of regulation and correlation, 

 which even determine the position and size of the young primordia. That the 

 size of the primordium is only of limited importance is shown by the fact that the 

 primordium of a prophyll may be caused to develop into a foliage leaf. 



According to Hartig (1. c.) and Moll (1. c.) the shortness of the basal inter- 

 nodes of a branch is due to fewer embryonic cells being devoted to their formation. 

 The unequal size of the adult internodes of Nitella and Chara is however obviously 

 produced by the different increase in length of the various segment-cells 5 . 



SECTION 3. Growth under Constant External Conditions (continued]. 



A brief description of a few typical instances of growth may be of 

 use to the general reader. For example, the different phases of growth 

 can best be traced upon a growing root-apex 6 by placing equidistant lines of 

 Indian ink upon it, and observing the rapidity and amount of growth in 

 the regions thus distinguished (Fig. i). It can then be seen that the 

 activity of growth in the segment-cells increases, until, owing to their own 

 growth and that of the parts in front of them, they become from four to 



1 Cf. Hartig, Lehrb. d. Anat. u. Physiol., 1891, pp. 259, 267. 



2 Vochting, Organbildung im Pflanzenreich, 1884, II. Th., p. 127; Mobius, Lehre von d. 

 Fortpflanzung der Gewachse, 1897, p. 88. 



3 This was known to Hales, Statics, 1784, p. 184, and was further studied by Moll, De invloed 

 van celldeeling en cellstrecking op den groei, 1876. Cf. also Wiesner, I.e., p. 467; L. Monte- 

 martini, Ricerche intorno all' accrescimento delle piante, 1897, p. 6 (Atti dell' istituto Botanico dt 

 Pavia) ; Biisgen, Bau u. Leben der Waldbaume, 1897, p. 6. The variation in the size of leaves was 

 known from Goethe onwards. 



4 Sanio, Jahrb. f. wiss. Bot., 1872, Bd. vil, p. 402; Hartig, 1. c., p. 286; Busgen, I.e., 

 pp. no, 116. 



5 Cf. also Askenasy, 1. c., 1878, p. 30. 



6 Sachs, Arbeit, d. Bot. Inst. in Wiirzburg, 1873, pp. 414, 590. The main features were 

 correctly determined by Ohlert, Linnaea, 1837, Bd. XI, p. 615; Wigand, Bot. Unters., 1854, p. 159; 

 Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. Ill, p. 96; Frank, Beitrage z. Pflanzenphysiol., 1868, 

 p. 34; N. J. C. Miiller, Bot. Ztg., 1869, p. 387; 1871, p. 727; Cisielski, Cohn's Beitrage, 1871, 

 Bd. I, II, p. 3. Cf. also Wettstein, Sitzungsb. d. Wiener Akad., 1884, Bd. LXXXIX, Abth. i, p. 59. 



