OSMOSIS 197 



The turgor of plant cells does not remain constant. The 

 concentration of the cell sap (therefore its osmotic pressure) 

 and the permeability of the protoplasmic (or vacuolar) membrane 

 undergo changes from time to time which appear to be capable 

 of "control" by the cell or by the plant as a whole. This is 

 probably true in the diurnal (day-and-night) and other move- 

 ments of leaves and flowers, where osmosis may be the operating 

 mechanism. An interesting case of the movement of leaves is 

 the closing of the leaflets of the sensitive plant, Mimosa pudica. 

 This plant closes its leaflets and drops its petiole when irritated 

 by a mechanical disturbance such as cutting or mere touching. 

 If the leaflet at the tip of a leaf is disturbed, it immediately 

 closes by folding close to the stem, and a moment later its mate 

 does likewise. Very soon, the adjoining leaflets close, then 

 those farther down the petiole, until all are folded. The stimulus 

 continues down the leaf stem until it reaches the base, where 

 there is a swelling called the pulvinus. The petiole then falls 

 until it hangs close to the main stem of the plant. The stimulus 

 does not stop at the base of the irritated leaf but turns up and 

 down the main stem to other leaves the leaflets of which close 

 in the reverse order from that of the first leaf. The manner in 

 which this stimulus is transmitted is one problem, but here we 

 are concerned with the mechanism closing the leaflets and 

 dropping the petiole. If the turgidity of the cells of plants is 

 reduced, the tissue collapses. When the pulvinus is rigid with 

 water, it holds the petiole and leaf up; but when pressure is 

 released, the tissue of the pulvinus becomes soft and collapses, 

 and the petiole falls. 



Plants grown experimentally in water cultures thrive best 

 when the salt solution is not only of the correct chemical constitu- 

 tion but also of the correct osmotic value. A solution hyper- 

 tonic to the sap will cause the cells to lose water; if too strong, 

 the cells may become plasmolyzed, resulting in injury and death. 

 A hypotonic solution is the natural one in the soil; plants may 

 then take in water rapidly. Variation in tonicity (osmotic 

 pressure) of soil water acts as a stimulus, and plants then do 

 better. Such changes constantly occur in the soil, owing chiefly 

 to rain. 



Marine organisms are bathed in an aqueous medium which is 

 of very constant composition. (Some variations and changes 



