196 



dammed up zvoter may come under the head of useful factors of cultivation. 

 At any rate inundation will always be more dangerous than those methods 

 of irrigation where the soil always remains accessible to the air. The oxygen 

 content of irrigation water increases oxidation in the meadow soils since 

 water filtering off through the soil shows a lesser amount of oxygen and, at 

 the same time, an increased amount of carbon dioxid and sulfuric acid in 

 comparison with water in use for irrigation^ So long as sufficient oxygen 

 is present the slow phenomena of oxidation of organic substances into 

 carbon dioxid, ammonia and nitric acid, which we term decomposition, are 

 accomplished chiefly by the action of micro-organisms. If a scarcity of 

 oxygen occurs, however, due to continued retention of the water, that 

 process of decomposition begins, partly of a purely chemical nature, partly 

 with the co-operation of bacteria, which we call decay, whose final products 

 are compounds which may still be oxidized. 



If the water accumulates in places where impervious layers of soil 

 entirely prevent any vertical flowing away and all horizontal flowing away 

 is also made difficult, the land becomes marshy. 



With the excessive wetting of the soil, the symptoms are again seen, 

 which usually appear gradually with root decay. In deciduous trees, 

 especially fruit trees, and w^ith grapes a premature yellow leaf (chlorotic) 

 condition becomes noticeable, wdiich advances from below upward. This 

 advancing death and falling of the leaves from the base of the branch to- 

 ward its tip bear witness tu the fact that the growing branches strip off their 

 older leaves in order to mature their younger ones, which happens also in a 

 gradual drying up. By this means, yellow leaves may be distinguished from 

 the pale leaves resulting from the action of frost, in which the young leaf 

 apparatus is disturbed and its normal chlorophyll action retarded. 



Conversion of Land Into Swamps. 



R. Hartig's- observations show that stagnant water is most injurious in 

 forest plantations since the sensitiveness of the trees to frost is increased 

 and freezing and heaving occur in the seed beds. Hartig^ observed decay 

 of the roots to a devastating extent in the tracts of the young pines in 

 Northern Germany. It begins between the 20th and 30th years when, after 

 a short period of weak growth, the trees, still covered with perfectly green 

 needles, topple over as soon as a weight of snow touches them or a high 

 wind acts on them. It is found that the tap root (see Growth of Stilts, p. 92) 

 is wet and rotted up to the base of the trunk while most of the lateral roots 

 appear to be healthy. vSuch a decay of the roots may indeed be found in 

 spruce plantations, but it is less noticeable because the superficially extended 



1 Wollny, E., Die Zei-setzung- der organi-schen Stoffe und die Humusbildungen. 

 Heidelberg 1897, Carl Winter, p. 3.=;i. 



- Hnrtig, R., liehrbuch der Pflanzenkrankheiten, 3rd. Ed. Berlin, Springer 1900, 

 p. 263. 



3 Die Wurzelfaule. Zonsetzungsersnlieiniingen des Holzes, Berlin, .Tul. Springer, 

 1878, p. 75. 



