^45 



the wilting of the leaves and their ultimate death, resulting from the de- 

 tention of the plant roots in water excessively charged with carbon dioxid, 

 Graebner cites Maxwell's experiments^ with citric acid and those of Tolf 

 and Blank with humic acids, all of which lead to similar results. This is the 

 place to record Ramann's statement as to the cause of retarded diffusion in 

 acid soils. Either the colloidal composition of the moor-substances can re- 

 duce the capacity for diffusion and the colloidal substances are precipitated 

 by neutralization with lime, or some direct action of the humic acids is 

 present. If one thinks of the discoveries showing the influence exerted by 

 slight acid increases on the protoplasm^, whereby its currents are arrested, 

 one must consider the direct action of the acid to be of the chief importance. 

 Special proof already exists of the retarding of transpiration by acids (tar- 

 taric, oxalic, nitric and carbonic acids, etc.) and its hastening by alkalis 

 (potassium, sodium, ammonia)^. It can therefore be said, with Schimper, 

 that plants in a strongly acid soil will suffer from physiological drought even 

 in the presence of abundant water. To this must be added that the great 

 power of humus to retain water makes the mechanical withdrawal of the 

 water from the soil particles much more difficult for the roots than if in 

 sandy soil. Plants are found to wilt in peaty soil or loam with a percentage 

 of water sufficient to keep them perfectly fresh in sandy soils, as Sachs'* 

 experiment has already shown. 



All these injuries due to the soil find expression most of all in the culti- 

 vation of pines, which subject Graebner^ has treated with especial thorough- 

 ness. He found in young pine plantations, which had grown tolerably well 

 for some years, that the shoots formed in May at first developed normally, 

 but, with the appearance of the summer drought, became grayish green in 

 color. If the dry period continued, the shoots begin to curl, the needles of 

 the previous year became blunt and brown and in many cases the little trees 

 dried up in a few weeks. By digging in the soil, it was found that swamp 

 ore had been formed below the roots or even around the still rather slender 

 ones. 



To supplement his description, Graebner pictures in the figures here 

 reproduced root development on swamp ore soils. We see in figure 29, that 

 the strongest and longest roots are spread out not far below the surface of 

 the soil and parallel to it, so that its nutrition must take place through the 

 raw humus and the lead sand, which is poor in nutritive substances. Since 

 root development is greater in solutions poor in nutritive substances than in 

 concentrated solutions, this results in a wide reaching out of the root 

 branches, which, in the present case, according to Graebner, seem several 

 meters long and but little branched. The aerial axis, however, is scarcely a 



1 Journ. Ann. Chem. Soc. XX (1898) p. 103. 



2 Pfeffer, Pflanzenphysiologie II Vol. 1904, p. 798. 



3 Pfeffer, Pfianzenphysiologie I Vol. p. 231. 



4 Sachs, Handb. d. Exp.-Physiol. Leipzig, 1865, p. 173. 



5 Graebner, R., Handbuch der Heidekultur, Leipzig, 1904, W. Engelmann, p. 231. 



