284 ABSORPTION OF WATER 



immersed in water. Absorption of water by leaves under such conditions does 

 not occur through the stomates, but directly through the epidermal cells. 



The leaves and other aerial organs of plants may also absorb traces of 

 M^ater-vapor directly from the atmosphere when the latter is saturated or 

 nearly so, but the quantity of water obtained in this way is usually negligible. 



Tiny droplets of water are sometimes projected directly through the 

 stomatal pores as a result of splashing during heavy rains, or when leaves are 

 artificially sprayed. Partial or complete injection of the intercellular spaces 

 with liquid water is sometimes brought about in this way. Direct penetration 

 of water in this manner is most likely to happen in species with relatively large 

 stomates such as tobacco. 



Discussion Questions 



1. How would you demonstrate convincingly that the rate of absorption of 



water by a plant is markedly influenced by its transpiration rate? 



2. Suggest several reasons why relatively low soil temperatures result in low 



absorption rates even when the temperature of the aerial parts of plants 

 is relatively high. 



3. Under what conditions can the transpiration rate be accepted as a fairly 



accurate indication of the rate of absorption? Under what conditions not? 



4. All species of plants reduce the soil to approximately the same water con- 



tent when the rate of transpiration is slow yet some species are able to 

 deplete the soil of more water than others when transpiration is rapid. 

 Explain. 



5. Leaves of tomato plants often recover from wilting more rapidly if the 



stems are cut and placed in water than when the soil around the roots is 

 well watered. Does this imply that roots are less efficient absorbing 

 mechanisms than the cut end of a stem? Explain. 



6. Herbaceous plants with dead root systems absorb water readily from soils 



with a water content above the moisture equivalent, but quickly wilt if the 

 water content drops below this value. Similar plants with living roots, 

 under identical atmospheric conditions, often wilt in wet soils, but show 

 no signs of wilting in soils below the moisture equivalent. Explain. 



7. Why do tobacco plants often exhibit wilting immediately after a heavy rain? 



8. A laboratory determination shows the soil in a given field not to be down to 



wilting percentage yet the plants exhibit permanent wilting. Give possible 

 explanations for this discrepancy. 

 g. Why does immersion of potted plants of some species in cracked ice often 

 result in wilting while plants of other species can be similarly treated with- 

 out ever wilting? 



10. In a given soil under which of the following conditions would you expect 



the most extensive root development: (l) at the field capacity? (2) at a 

 water content considerably above the field capacity? (3) close to the wilt- 

 ing percentage? Explain. 



11. Why won't seeds germinate in a soil at the wilting percentage when they 



will absorb water from solutions with a diffusion pressure deficit cor- 

 responding to a value less than the wilting percentage (Table 17)? 



