^77 



then become evident, which can also be produced experimentally by the use 

 of highly concentrated nutrient solutions : — short internodes, smaller leaves, 

 short roots having a great tendency to decay, reduced production and trans- 

 piration. A further cause of wilting is a lowered soil temperature. If a de- 

 gree of heat is not reached which is required by a certain plant so that the 

 roots can begin absorbing the water, while the temperature of the air permits 

 evaporation by the leaf apparatus, this disturbed equilibrium between water 

 demand and supply makes itself felt by wilting. 



A special, not rare case, is the zvilting of hot bed plants when the pots 

 are cooled during the re-working of the hot beds or during transplanting. 

 Inexperienced gardeners then water the plants abundantly and the turgidity 

 is restored if the water, previously warmed, awakens root activity. By a 

 repetition of the cooling, the same experiment can be carried out until finally 

 the pot is overloaded with water and the roots break down from a lack of 

 oxygen. 



Another case of the wilting of potted plants was observed by Hellriegel. 

 He found that plants wilted in large pots, which held three or four times as 

 much water as small pots of plants of the same species, which did not wilt. 

 This circumstance is explained by the relative water content of the soil, 

 which in the smaU pots amounted to 14 to 20 per cent., while the absolute 

 larger quantities of water in the larger amount of soil in the large pots was 

 so disturbed that it represented only 11 to 15 per cent, of soil moisture. In 

 this case, absorption was made more difficult for the roots in the larger pots, 

 by the less easily transported water held more firmly in the capillaries of the 

 soil, so that evaporation was in excess. 



In contrast to this physiological wilting we might term mechanical wilt- 

 ing those phenomena due to an actual lack of soil moisture be- 

 cause the mechanical transportation of water slackens in the ducts. Nat- 

 urally with the great demand for moisture in the leaves and the scanty re- 

 inforcement in the ducts, the air content increases and in this increase of the 

 air content above a certain degree may be seen the arrest of the water cur- 

 rent in the axial organs, as Strasburger^ emphasizes. In this, the air in the 

 tracheal elements will be more dilute, as the transpiration and assimilation 

 on warm days- are stronger, and the result is that a moistening of the soil 

 becomes so much the more quickly effective. In general, watering exerts a 

 lesser influence, the greater the turgidity of the plant'^ The great tracheal air 

 dilution shows itself also in the well-known fact, that field plants, wilting 

 rapidly in hot weather, will stiffen from the dew on the soil at night, — 

 especially since leaf evaporation is repressed at this time. 



1 Strasburger, Ed., Ueber den Bau und die Verrichtungen der Leitungsbahnen in 

 den Pflanzen. Jena 1891. cit. Bot. Zeit. 1892, p. 261. 



2 Noll, Ueber die Luftverdiinnung in den Wasserleitungsbahnen der hoheren 

 Pflanzen. Sitzungsber. d. Niederrheinischen Ges. f. Natur- und Heilkunde. Bonn 

 1897, 11. p. 148. 



3 Chamberlain, Houston Stewart, Recherches sur la seve ascendante. cit. Bot. 

 Jahresb. 1897, p. 73, 



