SECT. 11 PHYSIOLOGY 185 



and pigment pass into the surrounding water, and as a result the slices lose their 

 rigidity and become limp. 



On the otlier hand, the internal pressure in the cells of Fungi, Bacteria, and 

 marine plants is increased when they are placed in a less concentrated food- 

 solution, e.g. transferred to fresh water ; this often leads to a rupture of the 

 cell-wall. 



Plasmolysis is of great scientific use since it allows us to measure by comparison 

 the pressure within vegetable cells. If a solution of potassium nitrate of a definite 

 percentage produces an osmotic pressure of 5 atmospheres, and this solution 

 is just suflicient to remove the turgescence of a cell, it is clear that the cell-sap can 

 attract water with an equivalent force and give rise to the same pressure. (That 

 the turgescence is overcome is estimated in the case of an elastic and distended 

 cell by the cessation of the elastic shortening.) Since a 1 % solution of 

 potassium nitrate gives rise to an osmotic pressure of 4| atmospheres, the internal 

 pressure due to turgescence can be easily calculated in atmospheres. It amounts as 

 a rule to not less than 5 atmospheres, but often reaches 10 and even 40 or more 

 atmospheres (cambial cells, 25 atmospheres ; nodes of grasses 40 atmospheres). 

 The highest of these pressures far exceed the pressure of the steam in steam 

 engines. The pressure is increased when the plants live in concentrated solutions 

 instead of in water ; for in these conditions also the cells exhibit an internal 

 pressure. Thus the cell is able to regulate its osmotic pressure. The 

 mechanical force required to stretch a wilted or plasmolysed elastic organ to its 

 former length may also be used to give an approximate measurement of the force of 

 turgor in the organ wlien turgescent. 



Dissolved Substances. — If the protoplasm is really quite im- 

 permeable to the salts that have been considered above, not even 

 traces of them can enter the cell-cavity. Practically, however, the 

 impermeability of the protoplasm is perhaps not absolute for any 

 substance ; there are all grades, from substances that pass through 

 the protoplasm as easily as water, to those that are almost incapable 

 of passing through it. Alcohol, ether, chloral hydrate, also numerous 

 organic pigments, and, lastly, very dilute acids and alkalies diffuse 

 with special rapidity. 



The permeability of the protoplasm is not always the same, and 

 may be regulated according to the requirements of the cell. The 

 absorption or not of a substance is determined not by the whole 

 protoplasm but by its external limiting layer. In the further 

 passage of the substance, from the protoplasm into the cell-sap, the 

 wall of the vacuole exercises a similar power of selection, which in some 

 circumstances may differ from that of the external limiting layer. 

 On account of the selection exercised by the limiting layers of the 

 protoplasm, it is possible for the contents of the cell to be quite 

 different chemically from the surrounding medium. The cause of the 

 SELECTIVE POWER, by reason of which different cells can appropriate 

 quite distinct constituents from the same soil, is to be sought in this 

 most important property of the limiting layers of the protoplasm. 



From the same soil one plant will take up chiefly silica, another lime, a third 

 common salt. The action of Sea-weeds in this respect is especially instructive ; 



