196 PROTOPLASM 



The Animal Cell as an Osmotic System. — Osmosis was 

 originally, and has long remained, a botanical problem, though 

 it was soon taken up by the chemists. It has never been a leading 

 question in animal physiology, but it has been studied there by 

 Lucke and others. What little turgidity animal cells possess 

 (animal tissues are usually flabby) has been attributed to imbi- 

 bition pressure, such as makes a block of swollen gelatin firm, 

 rather than to osmotic pressure (or turgor). Nevertheless, 

 Lucke measures what he terms the turgor of animal cells (eggs of 

 echinoderms and red blood cells) and finds them to be perfect 

 osmometers, obeying the law of Avogadro (the law of equimo- 

 lecular concentration). 



The Role of Osmosis in Life. — Osmosis maintains the turgidity 

 or firmness of plant tissue and thus keeps the plant supplied with 

 water for metabolic processes. When turgor is normal, plant 

 tissue is firm. When the water supply is lowered and turgor 

 reduced, the tissue becomes flabby, and the leaves droop. This 

 is wilting. The amount of water which plant tissue can hold is 

 strikingly illustrated in the succulents — the extremely juicy 

 plants. These have spongy stems, like cacti, or fleshy leaves, 

 like those of Sedum, which are firm and "plump" with water 

 retained under pressure. Barrel cacti are cut by Indians to 

 obtain water for drinking. 



Osmosis contributes to the maintenance of the water supply in 

 plants through the so-called transpiration stream. The upward 

 flow of sap against gravity in trees is one of the great problems 

 in botany. There is apparently an osmotic gradient in plants, 

 with highest osmotic pressure in the leaves and lowest in the 

 roots. But if the flow of sap is by osmosis, it must be through 

 living cells, as only they, with their semipermeable protoplasmic 

 membranes and vacuoles, are osmotic systems. Actually, the 

 sap appears to flow through dead wood (xylem) most of the way. 

 Water does, however, first enter the plant through living cells 

 (root hairs), so that at least the first movement of water into the 

 plant is a purely osmotic phenomenon. Water continues to 

 diffuse from the soil into the living plant cell as long as the percent 

 of soil moisture is sufficient to keep the concentration of the sofl 

 solutions hypotonic to (of a concentration less than that of) the 

 concentration of salts and sugars in the cell vacuole. 



