120 APPLIED BIOLOGY 



such plants do not appear to take oxygen in the daytime from 

 the surrounding air, as they do at night, and as plants with- 

 out chlorophyll do both day and night. 



All plants produce excretions by the oxidation going on in 

 their cells. Most prominent of these is carbon dioxide, 

 which all plants give off at night to the air or water in which 

 they live. Plants without chlorophyll give off carbon dioxide 

 in daylight also ; but the green plants use this gas so rapidly 

 when making starch that during the daytime none appears 

 to be given off to the surrounding air. 



All plants have assimilation or constructive metabolism 

 of some foods into new protoplasm. This takes place only 

 in living cells. Much of the food containing only carbon, 

 hydrogen, and oxygen (i.e., carbohydrates and oils) is be- 

 lieved to undergo oxidation in cells or is stored for future use, 

 but does not become protoplasm. Probably only a small 

 part of the contents of an ordinary plant cell is living matter, 

 and much of the cell-substance which we see with the micro- 

 scope consists of food-materials, water, and other lifeless 

 substances. 



Digestion of foods is necessary whenever insoluble foods 

 (starch, oil, proteins) require transfer from cell to cell or to 

 distant organs of the plant. 



Moving liquids in the higher plants serve to transport the 

 foods, oxygen, and excretions ; but these liquids do not make 

 a complete circuit as do the animal circulating liquids (blood 

 and lymph), whose function is also transportation of foods, 

 oxygen, and excretions ( 52). 



Some form of irritability, or power of responding to stimuli, 

 is present in all plants. But there are no special nervous 

 organs, such as are connected with irritability in higher 

 animals. 



All plants have the power of reproduction, either from parts 

 of their bodies which can grow into complete plants (asexual 

 reproduction), or from egg-cells which usually require union 



