I 4 GROWTH 



The total contraction may amount in the course of two or three 

 weeks to 10 per cent, of the length of the radicle of the beet-root, and 

 25 per cent, of the length of that of the clover l . In the case of the roots 

 of Arum maculatnm and Agave americana, a shortening of as much as 

 50 per cent., and in those of Oxalis clegans even as much as 70 per cent., 

 of the lengths of the roots has been observed 2 . A slight shortening 

 was also observed by Rimbach in the hypocotyls of a few seedlings 

 (Taraxacum, Atropa Belladonna)^, and a more pronounced one by Berthold 

 on the rhizoids of Antithamnion crticiatum 4 . In the latter case the filaments 

 grow apically, and the segment-cells subsequently broaden and decrease in 

 length by from 30 to 50 per cent. 



By contractions of this kind the plants can be drawn deeper into the 

 soil as soon as the younger parts of the roots have become sufficiently 

 firmly attached, and indeed it may happen that the seedling is even drawn 

 beneath the surface. It is by the annually repeated contraction of the 

 roots that those rhizomes, bulbs, and tubers which grow vertically or 

 obliquely upwards are kept imbedded in the soil. In order that the same 

 constant depth may be maintained, a certain power of regulation is 

 necessary, which is attained in the cases mentioned either by the non- 

 formation of the contractile roots under the conditions existing beneath a 

 certain depth, or by their assuming a horizontal direction of growth. Many 

 plants normally produce contractile and non-contractile roots at the same 

 time, while in others only the latter variety are formed 5 . 



In the rhizoidal cells of Antithamnion it can be directly seen that the short- 

 ening is due to an active change of shape of living cells. In contractile roots 

 the active cells are present in the inner cortex, and, according to de Vries, also 

 in the central pith of fleshy roots. These cells shorten as they broaden, but 

 their tendency to change of shape acts against, and is partially antagonized by, 

 the wood-cylinder and epidermis to which the contractile tissues are attached. 

 The wood-cylinder and epidermis are at first stretched by the actively growing 

 cortex and pith, but when the latter tissues shorten, the wood and epidermis are 

 subjected to longitudinal compression. These alterations in the tissue-strains can 

 be seen by isolating the respective tissues and noting their changes of shape. They 

 are also frequently indicated in the intact root by the folding and crumpling of the 

 outer surface, and by the wavy curvature of the vascular cylinder. If the turgor 

 of the thin-walled active cells is removed by plasmolysis, the root elongates slightly. 



1 De Vries, Landw. Jahrb., iSSo, Bd. ix, p. 37 ; 1879, Bd - VIII > P- 474? l8 77> Bd - VI > P- 9 28 - 



2 Rimbach, Fiinfstiick's Beitr. zur wiss. Bot., 1897, Bd. XI, p. i ; 1899, Bd. xm, p. 20, where the 

 remaining literature is given. The drawing of seedlings into the soil was observed by Tittmann, 

 Flora, 1819, Bd. n, p. 653. 



3 Cf. also Jost, Bot. Ztg., 1890, p. 435. 



4 Berthold, Jahrb. f. wiss. Bot., 1882, Bd. xm, p. 607. 



s For details see Rimbach, 1. c., p. 15. [Went observed no shortening in the aerial roots of 

 Aroids (Ann. du Jard. Bot. de Buitenzorg, 1895, T. i, p. 19), whereas a slight contraction may occur 

 in those of Vanilla aromatica and other orchids.] 



