THE IMPORTANCE OF THE ROOT-SYSTEM 155 



cultures develop badly or not at all in soil over-saturated with water, 

 and this is probably due to insufficient aeration, that is to a deficiency 

 of oxygen l . 



In water-cultures, there is necessarily a certain concentration at which 

 the optimal development of the root-system takes place, for in too dilute 

 a solution active growth is impossible, while, when the concentration is 

 excessive, growth is inhibited and finally ceases. Nobbe 2 found that, with 

 Barley and Buckwheat, the best development of the root-system was produced 

 in a nutrient solution containing from one-half to two parts of inorganic salt 

 per thousand of water, while in a solution of ten per thousand, the lateral 

 roots were unable to continue their development. 



For similar reasons, too high or too low a percentage of dissolved salts 

 in the soil must hinder development, and hence in a very poor soil the 

 root-system develops badly 3 . In producing such results other special 

 actions apparently come into play : thus roots which grow through 

 alternating layers of sand and earth branch more markedly in the latter. V 

 This can only be due to a local stimulus exercised either by the nutriment 

 as a whole, or by certain special substances, for the contact with solid 

 substances and the presence of water act in the layers of sand also, as well as 

 in a layer of humus poor in nutrient salts, in which the branching is similar 

 to that in sand. That specific chemical stimuli may influence growth is 

 well known, and indeed without such a stimulus the seeds of Orobanche 

 cannot germinate (cf. Sect. 64). It is obviously of the utmost advantage 

 in the economy of the plant that a richer soil should induce a better 

 development of the root-system. 



Nobbe 4 performed similar researches with Maize and Clover, employing layers 

 of the same soil placed in a box with and without previous saturation with nutrient 

 material. In the soil with less nutriment, the development and branching of the 

 root-system was less marked. The same was observed by Thiel and by Hoveler in 

 alternating layers of sand and humus. The absence of a single essential nutrient 

 substance apparently brings about a similar result, and this is shown by certain of 

 Frank's researches, in which one portion of the root-system of the same plant 

 (Maize and Peas) grew in soil containing nitrates, the other in soil in which no 

 nitrates were present 5 . 



1 Cf. Waeker, Jahrb. f. wiss. But., 1898, Bd. xxxn, p. 71. 



J Nobbe, Versuchsst., 1864, Bd. vi, p. 22 ; Wieler, Bot. Zeitg., 1889, p. 550. In some cases, 

 roots which pass from the soil into water become markedly elongated, as Duhamel first noticed 

 (Naturgesch. d. Baume, 1764, Bd. I, p. 107). 



3 Cf. Fr. Schwarz, Zeitschr. f. Forst- u. Jagdwesen, 1892, p. 89 (for Pinus sylvestris). Knight 

 (Phil. Trans., 1811, p. 211} long ago observed that roots grow more strongly in good soil. 



* Nobbe, Versuchsst., 1862, Bd. iv, p. 217, and 1868, Bd. X, p. 94. Similar experiments by 

 Stohmann, Jahresb. d. Agr.-Chemie, 1868-9, p. 242. 



5 Thiel, quoted by Sachs, Exp. Physiol., 1865, p. 178; Hoveler, Jahrb. f. wiss. Bot., 1892, 

 Bd. xxiv, p. 294 ; A. B. Frank, Bot. Zeitg., 1893, p. 153. 



