154 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



functional activities of the root and shoot systems. The manner in which 

 external conditions affect growth will first be dealt with subsequently, and 

 at the same time it will be shown that a modification of growth is often 

 the result of the action of different and varying factors, and that these 

 commonly cannot be clearly distinguished from one another. In addition 

 to the direct and indirect mechanical effects already noticed, the growth 

 and development of the roots and all underground parts are dependent upon 

 the amount of water present in the soil, while the branching throughout 

 the soil is influenced by the amount and character of the food material, 

 as well as by the quality and concentration of the water of the soil, by 

 the aeration of the soil, by its temperature, and in certain cases by the 

 penetration of light. The power of reaction and accommodation to these 

 and other conditions is naturally of the utmost importance, and it is well 

 known that very many plants grow, and find all the conditions necessary to 

 their existence, in soils and media of widely different character. 



The resistance of an ordinary soil and even of stiff clay is in general 

 easily overcome by the roots, but when the resistance offered is considerable, 

 the plastic root apices bend to one side, and by continued growth in length 

 may creep round the obstacle. If no lateral curvature is possible, the turgid 

 and swollen root apex may burrow its way through soft tufa (solidified 

 volcanic mud) or even through the tissues of a living plant l . 



In potted plants the centrifugally growing roots apply themselves closely to the 

 sides of the pot with which they come in contact, and thus form against it a closely 

 felted mass of roots. In spite of this, such plants have subservient to them a much 

 smaller area of soil than others growing in the open, and hence old pot plants, which 

 have fallen off in vigour, may be strengthened by removal to a larger pot, or by 

 giving them a supply of nutrient salts 2 . Such plants also become stronger if the 

 roots are allowed to grow through the hole at the bottom of the pot into the soil 

 beneath, and this method of cultivation is often well adapted to obtain strong 

 plants for experimental purposes. 



Since growth is retarded in dry soils, the root-system develops more 

 markedly in the moister layers, to which young roots are attracted 

 by their hydrotropism. It is the action of moisture which induces the 

 development of roots on the rhizophores of Selaginella, and no doubt 

 it is for a similar reason that aerial roots branch vigorously when they 

 penetrate the ground 3 . In some cases, plants which grow well in water- 



1 Details by Pfeffer, Druck und Arbeitsleistungen, 1893, p. 362 ; Peirie, Bot. Zeitg., 1894, p. 169. 

 On the rhizoids of mosses, see Haberlandt, Jahrb. f. wiss. Bot., 1886, Bd. XVII, p. 476; Hb'veler, 

 ibid., 1892, Bd. xxiv, p. 296. On the penetration by fungi, &c., see Sect. 65. 



a Lindley, Theory of Gardening, 1842, p. 189; Sachs, Flora, 1892, p. 173; C. Kraus, Forsch. 

 a. d. Geb. d. Agr.-Physik, 1894, Bd. XVII, p. 55. 



3 Pfeffer, Arb. aus Wurzburg, 1871, Bd. I, p. 97. See Schimper, Bot. Centralbl., 1884, Bd. xvn, 

 p. 285, and Bot. Mitth. a. d. Tropen, 1888, Heft 2. 



