82 



METABOLISM 



both of higher and lower grade, which frequently thrive better in a weak alka- 

 line than in weak acid solutions (BENECKE, 1898). In addition to the reaction 

 the concentration and quantities are of primary import. In general we use 

 a solution of 1-5 per cent, of salts ; but NOBBE (1867) found that a 5 per cent, 

 solution was injurious to barley, and was forced to use solutions of weaker 

 concentration and to employ larger vessels. More recently, WORTMANN (1892) 

 experimented with culture vessels of much greater capacity, holding about 

 25 lit. In such vessels plants grow remarkably well, and it is unnecessary to 

 renew the culture fluids during the vegetative period. 



Annual plants are especially well adapted for water-culture experiments, 

 since rapid results may be obtained from them. 

 Thus many grasses, Cruciferae, buckwheat, rape, 

 linseed, and Tradescantia, as well as potatoes, may 

 be employed with success. Indeed in the case of 

 buckwheat it has been affirmed (NoBBE, 1868) 

 that the plant grows better and shows a far greater 

 increase in dry weight (from I to 4786) under such 

 artificial conditions than when cultivated in soil 

 in the ordinary way. A similar remarkable develop- 

 ment is attained by oats, which, according to 

 WOLFF (1868) show an increase from i to 2359. 



Many trees also, such as the oak, horse-chest- 

 nut, and alder, can be cultivated in aqueous solu- 

 tions. It is not surprising, however, to find that not 

 every plant proves a suitable subject for water- 

 culture, since the method assumes a capacity for 

 absorption on the part of the root under conditions 

 which are far from natural a capacity which the 

 root of every plant does not possess. In those 

 cases in which the water-culture method is un- 

 favourable to the growth of the plant, the method 

 recommended, especially by HELLRIEGEL (1883), 

 may be adopted, viz. to employ a medium consist- 

 ing of a quantity of sand which has been thoroughly 

 cleaned by being heated to redness and boiled in 

 sulphuric acid, and to which has afterwards been 

 added the ingredients whose functions require 

 investigation. 



The culture methods used for the lower plants, 

 Algae and Fungi, need not be studied here in 

 detail, because they are either self-apparent or 

 are, as in the case of seaweeds, still in want of 

 improvement. 



Collecting together all the results which have 

 been arrived at as to which constituents of the ash 

 in different plants are indispensable, we find that 

 the six elements essential, according to BIRNER and LUCANUS, to the growth of 

 oats, are also essential to all other Phanerogams, or, at any rate, to the majority 

 of them, while to buckwheat (according to NOBBE, 1862) chlorine is also essen- 

 tial. Since, in other cases also, chlorine has been found to produce a favourable 

 effect it may be added to the nutritive solution. The problem as to what special 

 demands on inorganic materials individual plants may make will be discussed 

 later on. It has been shown by BENECKE (1894-8) and MOLISCH (1895-6) 

 that plants of low grade such as Fungi and Algae require fewer inorganic salts 

 than Phanerogams, for calcium is not essential in their case, so that only 

 five elements have to be considered as essential. 



Fig. 19. Buckwheat grown in water- 

 culture solutions. /, normal ; //, with- 

 out potassium. (After NOBBE, from the 

 Bonn Textbook.) 



