12 



GROWTH 



a plaster of Paris cast, only the meristematic apex remains capable of 

 growth l . 



Changes must also occur whenever the relationship between the 

 vegetative and elongating zones is disturbed, whether by internal or 

 external agencies. Thus the length of the growing zone usually increases 

 when growth becomes more active 2 , and also when the stem is etiolated 3 , 

 while the temperature, supply of water, and other external factors may 

 also influence it 4 . 



All growing zones must be sufficiently protected from unfavourable 

 external influences, for soft meristematic tissues are especially liable to 



mechanical injury, and also readily shrivel 

 when exposed to dry air. Hence the 

 embryonic tissues in the bud are protected 

 by the sheathing leaves, as is also the 

 cambium cylinder by the layers of bark 

 or cortex which enclose it. Similarly the 

 root-cap shields the growing apex of the 

 root as it grows between and forces apart 

 the particles of soil. Indeed a protective 

 covering is seldom absent from large 

 growing apices, although the more minute 

 growing ends of hairs, fungal hyphae, and 

 rhizoids are naked, and a few roots even do 

 not possess a root-cap. Growing apices of 

 small diameter are relatively more resistant 

 than larger ones, and hence on this account 

 alone are more capable, even when without 

 any special protection, of withstanding the 

 different external influences to which they 

 are exposed. 



Intercalary vegetative zones can with- 

 stand direct pressure amounting to from 

 3 to 5 or more atmospheres, but must 

 also be sufficiently strengthened to bear the strains and torsions due to the 

 total weight of leaves and stem above them. This is commonly attained 

 by the development of investing sheaths, which serve at the same time as 



FIG. 5. Portion of stem of Secah cereale. 

 A) nat. size. A portion of the leaf-sheath j 

 has been removed as far as the node k. B^ 

 longitudinal section through the node k 

 magnified four diameters ; z>, growing inter- 

 calary zone at base of internode. 



1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, pp. 352, 381. 



2 Hofmeister, Allgem. Morphol., 1868, p. 421 ; Sachs, Flora, 1873, p. 322 ; Askenasy, 1. c., p. 74 ; 

 Wettstein, Sitzungsb. der Wiener Akad., 1884, Bd. LXXXIX, p. 92 ; Rothert, Cohn's Beitrage z. 

 Biologic, 1896, Bd. VII, p. 125 ; Montemartini, Ricerche intorno all' accrescimento d. piante, 1897, 

 p. 9 (Atti d. Istituto Bot. d. Pavia). Cf. also Errera, Bot. Ztg., 1884, p. 536. 



3 Cf. Strehl, Unters. ii. d. Langenwachsthum d. Wurzeln, 1874, pp. 15, 21. 



* Cf. Askenasy, Ber. d. Bot. Ges., 1-890, p. 82 ; Popovici, Bot. Centralbl., 1900, Bd. LXXXI, 

 P. 33 



