THE ROOT 259 
Turcor.—If the cell sap of the root-hair or any other plant 
cell becomes concentrated above that of the soil solution, the cell 
is caused to bulge. ‘This bulging is a manifestation of osmotic 
pressure within and the condition resulting is termed ¢urgor. 
Cells of plants exhibiting this phenomenon are said to be in the 
state of turgescence or rigidity. Since turgor is coexistent with 
growth, conditions affecting turgescence affect at the same time 
all growth processes. 
PLAsMOLysIs.—If root-hairs or other plant cells are placed in 
solutions of a greater density than that of their cell sap content, as 
for instance 5 to 10 per cent. salt or sugar solution, exosmosis 
becomes more rapid than endosmosis. The cell sap is extracted 
more rapidly than the fluid enters from without, with a result that 
the protoplasm is loosened from the cell wall and caused to 
collapse. This condition is called plasmolysts and the cell is said 
to be plasmolyzed. Plasmolysis throughout a tissue or organ 
results in wilting. Many plants wilt on account of too high a 
concentration of soil solutes. 
ABSORPTION OF NUTRIENT SALTs.—The plasma membranes of 
all living plant cells are permeable to certain solutes (substances 
in solution) and impermeable to others. ‘These membranes in 
the root-hairs absorb each solute particle separately and accord- 
ing to the need or attraction for that substance; this is called 
selective absorption. 
The higher green plants vary greatly in respect to their 
mineral requirements. Of thirty-one elements found in the ash 
of plants, only eleven occur regularly, ¢.g.—sulfur, chlorine, 
phosphorus, silicon, potassium, sodium, calcium, magnesium, 
iron, aluminum (a_trace) and manganese (a trace), Ten 
elements are absolutely required in the form of water-soluble salts 
for the normal growth of higher green plants. These ten are 
termed essential elements and comprise the following: nitrogen, 
sulfur, phosphorus, calcium, potassium, magnesium, tron, manganese, 
boron and zinc. Traces of copper have also been found essential 
for the growth of some plants. 
IMPORTANCE OF EssENTIAL ELEMENTS.— Nitrogen, sulfur and 
phosphorus are constituents of proteids and so of protoplasm and 
are essential to their formation. Without sufficient nitrogen, 
