152 



The production of carbon dioxid forms a measure of the energy pro- 

 duced by a root in raising water, boring into the soil and other life-functions. 

 Kossowitsch has furnished quantitative determinations on this points He 

 found that mustard plants in water cultures assimilated about three times 

 as much carbon for the life processes of their roots, as was necessary for 

 the formation of the roots themselves. 



The strength of the root activity, especially in lifting water, might de- 

 pend also on the differences in temperature between the atmosphere and 

 the soil. The greater this difference, the more energetic the work done. 

 MacDougal's- experiments in the New York Botanical Gardens prove how 

 great such differences may be. He found in June, that the soil temperature 

 at a depth of 30 cm, was at times 20°C. lower than that of the air. Naturally 

 the water content of the soil here becomes a decisive factor and the dift'er- 

 ences decrease as the soil becomes drier and more accessible to the air. The 

 moisture holding capacity and, in sandy soils, the amount of production will 

 depend, in the same soil, on its granular structure and will be the greater as 

 the sand is finer grained. Livingston and Jensen^ experimented on this 

 subject. They cultivated different plant species under similar conditions, in 

 soils which contained admixtures of different sized quartz grains in the 

 different experimental series. It was shown that the best growth always 

 occurred where the quartz sand was very fine. 



By means of the above observations we get an insight into the distur- 

 bances which must take place, in the activity of the roots, if the water supply 

 of a region is less, because the ground water level has been lowered. An 

 old tract of trees survives, because part of the deep growing roots reach 

 the ground water level and are able to compensate the loss by evaporation 

 of the tree crowns, when the soil water is reduced to a minimum during 

 periods of extended dryness. The roots lying in the earth, permeated by the 

 ground water, are adapted to these conditions. When these roots are per- 

 manently exposed to drought they are destroyed or function feebly. Not 

 only the economy of the tree suffers from the insufficient water and food 

 supply, but even the soil itself, since, entirely aside from the paralysis of 

 bacterial activity, the secreting ability of root hairs and tips affecting the 

 decomposition of the soil also ceases. The soil becomes "lean" and the 

 trees begin to show dead branches in the periphery of their crowns. Since 

 parasites settle on dying parts completing the destruction of the tissues, 

 this blight of the tree tops is explained in the majority of cases as a purely 

 parasitic disease and treated as such. 



1 Kossowitsch, P., Die quantitative Bestimmung der Kohlensaure, die von 

 Pflanzenwurzeln wahrend ihrer Entwicklung ausgeschieden wird. (Russ. Journal f. 

 experim. Landwirtschaft, 1904, Vol. V, cit. Centralbl. f. Agrikulturchemie, 1905, 

 Part 6, p. 367). 



2 MacDougal, D., Soil Temperatures and Vegetation. Repr. Monthly Weather 

 Review for August 1903, cit. Just, Bot. Jahresb 1903, II, p. 557. 



3 Livingston, B., and Jensen, G., An Experiment on the Relation of Soil Physics 

 to Plant Growth. Bot. Gaz. Vol. XXXVIII, cit. Bot. Centralbl. 1904, No. 50, p. 617. 



