280 Biology in America 



mosis, or the passage of solutions through membranes must 

 occur. But if the shell be chipped off from both ends of a 

 hen's egg, the shell membranes being left intact at one end, 

 and the yolk and white removed from the other, into which a 

 glass tube is sealed Avith a few drops of sealing wax; and if 

 now the egg be filled with a solution of sugar, and then im- 

 mersed in water, until the water is at the same level with the 

 solution in the tube, the latter will soon be seen to rise due to 

 the passage of water through the egg membrane into tlie sugar 

 solution; while more slowly the sugar will ditt'use in the re- 

 verse direction. Here we see in non-living matter the same 

 phenomenon of osmosis, which is so fundamental a factor in 

 all living processes. 



In the exchange of materials between the cell and its 

 environment, its membrane determines what substances shall 

 enter and leave the cell. Thus an uninjured beet may be 

 placed in water without losing any of its color. But cut 

 the beet and its color readily diffuses outward. So in the 

 absorption by roots of substances from the soil and by the 

 walls of the intestine from the digested food stuffs, the cell 

 membrane exercises what is known as "selective absorption," 

 taking some and rejecting others. In the passage of sub- 

 stances between mother and child, through the walls of the 

 placenta, the cells of the latter exercise a selective function, 

 allowing food materials and oxygen to pass from mother to 

 child, and waste materials to pass in the reverse direction. 

 This selective activity of living membranes is strikingly 

 shown by experiments on barley seeds, which are not killed 

 by sulphuric acid because it cannot penetrate them, but are 

 destroyed by bichloride of mercury, which readily enters. 



In the burning coal of the furnace and in the forest's 

 decaying logs, one of the final products of combustion or 

 decay is carbon dioxide. So too when we exhale the carbon 

 dioxide from our lungs we are casting off one of the end 

 products in the combustion or oxidation of our foods and 

 our tissues. 



Throughout the entire process of metabolism, of growth, 

 repair, decay, the body of animal or plant is a physico- 

 chemical laboratory in which are taking place the processes 

 of the non-living world. 



Another characteristic feature of living things is their 

 power of movement. This is not evident at firet sight in 

 all organisms, notably plants. In fact, one of the criteria 

 formerly presented as distinguishing plants from animals was 

 the fixity of the former as compared with the motility of the 

 latter. This distinction we now know to be false however, 

 for even in the apparently non-motile plants there is circu- 



