138 



SEWAGE AND ITS PURIFICATION 



Therefore in one hour a young leaf of a cereal can evolve about 

 its own weight of water. Hellnegel and Wollny found that 

 the transpiration of water varies from 233 to 912 pounds for 

 every i pound of plant tissue formed, according to the leaf 

 surface and length of growth, being greatest in clovers and 

 grasses, and least in roots and potatoes. Lawes estimated that 

 250 to 300 parts of water are evaporated for every i part of 

 dry solids elaborated by the plant. The transpiration of 

 Graminese (grasses and cereals) is greater than other plants; 

 hence they are indicated as absorbing a larger quantity of 

 sewage. At the Berlin sewage farms the proportion grown is, 

 in acres, cereals 3,000, grass (rye grass and Timothy) 2,000, 

 root crops 1,000, oil seeds (colza, etc.) 250, with rotation. 



Italian rye grass, according to Rawlinson and Read's report 

 to the Local Government Board in 1876, " absorbs the largest 

 volume of sewage, occupies the soil so as to choke down weeds, 

 comes early into the market, bears five to seven cuttings in the 

 year, and produces 30 to 50 tons of wholesome grass per 

 acre." It generally exhausts itself in about three years, when 

 it is ploughed up and replaced by root crops (usually mangolds) 

 or cabbages (Beddington), with a return afterwards to rye 

 grass. Mangolds yield a heavy but rather watery crop. Wheat 

 and oats are stated to run to straw rather than to grain. 

 Leguminous plants, which are capable of taking up nitrogen 

 from the air, are not adapted for an object which aims at re- 

 ducing the organic nitrogen. The Royal Agricultural Society's 

 Report on the Bedford Sewage Farm mentions one plant, 

 *' prickly comfrey," useful for horse fodder, "which it seems 

 impossible to damage by sewage, as it was completely flooded 

 for three weeks in succession, with benefit, and yielded three 

 crops in a year." Celery also flourishes, and sunflowers have 

 been successful. Generally the plants that are found to suit 



