250 A TEXT-BOOK OF BOTANY. 



leaves the acid substances are decomposed and to some extent 

 dissipated in the form of gases. 



The chief agents in the decay of leaves are fungi and bacteria. 

 A number of forms of animal life also contribute greatly to the 

 decay, as earthworms, larvae, flies and beetles and myriapods 

 or thousand-legged worms. Coville distinguishes two kinds of 

 leaf mold: (o) In which the leaves show a condition of imperfect 

 decomposition, due to the development and maintenance of an 

 acid condition, which is inimical to the growth of microorganisms 

 of decay. Because of the resemblance of this mat or turf to bog 

 peat in appearance, structure, and chemical composition, and be- 

 cause it supports a type of vegetation similar to that of bog peat, 

 it has been named upland peat. This is characteristic of the 

 sandy pine and oak woods, where grow huckleberries, laurel, 

 pipsissewa, pink lady's slipper, trailing arbutus, etc. (b) The 

 other is characteristic of the black mellow mold made up of com- 

 pletely rotted leaves, the acidity being neutralized in part by the 

 calcium present in the leaves and partly by the underlying soil, 

 which is usually of a calcareous nature. This is characteristic of 

 forests of tulip poplar, ash, and oaks, in which grow sanguinaria, 

 caulophyllum, hydrastis, trillium, etc. 



Organic Constituents in Soil. — During the past few years 

 Schreiner and his associates in the Bureau of Soils, U. S. Depart- 

 ment of Agriculture, have isolated and identified a number of 

 soil constituents. They have found that certain of these con- 

 stituents, as dihydroxystearic acid, are rather characteristic of 

 poor soils, and that the effect of fertilizers on such soils was to 

 increase their fertility by neutralizing their toxic constituents 

 rather than- by the addition of any food constituents to the soil. 

 The compounds isolated by them have varied considerably, and 

 may be grouped into the following classes: i, Paraffin hydro- 

 carbons, represented by hentriacontane ; 2, hydroxy- fatty acids, 

 represented by a-monohydroxystearic acid and dihydroxy- 

 stearic acid ; 3, organic acids of unknown constitution, represented 

 by agroceric acid, paraffinic acid, lignoceric acid, and a number of 

 resin acids ; 4, esters and alcohols, represented by agrosterol, phy- 

 tosterol, glycerides of fatty acids, and resin esters ; 5, carbohy- 

 drates, represented by pentosans and pentose sugars; 6, hexone 



