302 THE INTERNAL WATER RELATIONS OF PLANTS 



disturbances resulting from dehydration of the cell. Purely mechanical effects 

 such as pressure, stretching, tearing, etc. often are destructive to protoplasm. 

 During drying of cells the protoplasm is often subjected to just such effects. 

 The vacuole usually contracts more than the cell wall thus leading to distor- 

 tion and tearing of the protoplasm. Such drastic disturbances in the proto- 

 plasmic sj'stem usually result in its death. 



As shown by the same investigator cells with a small surface in propor- 

 tion to their volume and cells in which the size of the vacuole is small relative 

 to the protoplasmic mass are usually less subject to injury during desiccation 

 than cells of structurally opposite types. In cells which fall into one or both 

 of these classes dehydration results in relatively little mechanical deformation 

 of the protoplasm. 



In support of his views Iljin (1933) was able to demonstrate that many 

 types of plant cells, including even some from such parenchymatous tissues 

 as the leaf cells of lilac, could be slowly dried out until all water disappeared 

 from their vacuoles and subsequently restored to their turgid condition without 

 killing them. This seems to indicate that desiccation of the protoplasm per se 

 is not the cause of the death of plant cells. The turgidity of such cells can 

 be restored without injury only if they are allowed to imbibe water very slowly 

 from concentrated solutions. If immersed directly in water the ensuing rapid 

 absorption results in mechanical deformations of the protoplasm which cause 

 its disorganization. Death of otherwise resistant plant cells apparently may 

 be brought about either by a too rapid drying, or by a too rapid absorption 

 of water while in the desiccated state. 



Discussion Questions 



1. Why do light showers which do not result in penetration of water to the 



roots often result in restoring the turgidity of wilted plants? Would 

 this always occur? 



2. Why will a wilted potted plant often recover if placed in a saturated atmos- 



phere but sometimes not? 



3. Which of the osmotic quantities of the mesophyll cells of a maple tree will 



show the greatest variation during a twenty-four hour period in mid- 

 summer under "standard day" conditions? Why? 



4. Leaves of "shade" plants usually wilt upon a reduction in their water con- 



tent of 5 per cent or less, while most "sun" species do not wilt unless the 

 reduction in leaf water content is 20 per cent or more. Explain. 



5. Two similar twigs are cut from a tall tree, one from near the top, the second 



from one of the lower branches. Both are placed with their cut ends 

 in water. Which will at first transpire more rapidly per unit of leaf area? 

 Would the time of the day at which the twigs were cut make any dif- 

 ference in their relative transpiration rates? 



6. At 6 A.M. the water content of leaves on a certain species of plant is 



found to be 90 per cent; at 4 p.m. on the same day 88 per cent. In terms 



