86 



For every such external factor there is in fact for each in- 

 dividual plant a minimum., maximum, and optimum, degree of 

 intensity with reierence to its effect on any particular function, 

 the first two being the extremes beyond which the function 

 ceases to be performed and the last being that degree of inten- 

 sity under the effect of which the protoplasm is best able to 

 exercise the function in question. Green plants, for instance, 

 cannot manufacture their organic carbonaceous food from inor- 

 ganic materials without light, while if the light is too intense 

 the green chlorophyll, by means of which the manufacture is 

 made possible, is destroyed, so that the manufacture of food 

 is only possible provided that the intensity of the light is 

 suitable. Again if the temperature of the soil falls below 

 a certain minimum, or rises above a certain maximum, the 

 protoplasm in the root-hairs becomes inactive and the roots 

 can no longer provide the plant with the necessary water 

 and salts from the soil. 



83. The entire body of some 



minute green plants which live in water and are called alga3 

 consists of a single cell, such as has been described in Part II, 

 in which more or less of the cavity of the cell is occupied by 

 the so-called vacuolea, filled with cell-sap and surrounded by 

 a layer of protoplasm which lines the thin outer wall of 

 cellulose and in which are imbedded the nucleus and chlorophyll 

 corpuscles. Such a minute plant is just as capable of exercis- 

 ing the functions of nutrition, growth, respiration, and repro- 

 duction, as are plants the bodies of which consist of enormous 

 numbers of such cells and exhibit an elaborate differentiation 

 into ^rgans. Such a cell, for instance, is able to absorb 

 fro n the water in which it lives water with traces of mineral 

 salts in solution. This is effected by osmosis or diosmosis, 

 a phenomenon by means of which two different liquids, or gases, 

 which are capable of mixing and which are separated by a 

 thin membrane permeable to both of them, are able to diffuse 

 through the membrane until a condition of equilibrium is 

 attained, the liquid or gas on each side of the membrane then 

 being of the same density and composition. If the membrane 

 is more easily permeated by one solution than by the other a 

 larger quantity of the former will pass through it and the 

 latter will consequently increase while the former decreases in 

 volume. The current which flows towards the solution which 

 increases in volume is distinguished as endosmose and the cur- 

 rent in the opposite direction is exosmose. Endosmose usually 

 takes place towards the denser liquid and if a pig's bladder is- 



