400 READINGS IN BIOLOGICAL SCIENCE 



among plants, which changes starch, the stored-up food of the plant, into 

 one of the sugars. When the maple sugar sap is flowing in the spring we 

 know that a similar reaction has been taking place within the tree, and all 

 the beauty of the young spring's growth depends upon it. A similar reaction 

 takes place in our own mouth, under the influence of an animal ferment 

 known as ptyalin, and present in the saliva of many mammals. But a similar 

 result can also be obtained in the test tube of the chemist by boiling starch 

 in dilute acid. 



In the exchange of materials between the cell and its environment, its 

 (the cells) 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 diff^uses 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 substances 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 mate- 

 rials to pass in the reverse direction. This selective activity of living mem- 

 branes is strikingly shown by experiments on barley fruits, 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, 

 of body of animal and 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 move- 

 ment. This is not evident at first 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 circulation of cell sap, and move- 

 ments of leaves and roots in response to stimuli; while among animals, the 

 attached forms such as sponges, sea anemones, barnacles, etc., either lack 

 locomotive power or possess it in very slight degree. 



All living things then are motile to greater or less degree. But is this qual- 

 ity lacking in the non-living world? Place a diluted drop of ink under the 

 microscope and it becomes a microcosm of violent activity. Wind and 

 water are ever active. The earth is flying through space at the rate of i8^ 

 miles a second and the universe is a realm of external motion. Light and 



