130 APPLIED BIOLOGY 



stances. Thus the enzyme (diastase), which digests starch 

 to sugar in plant cells, is very similar to enzymes secreted in 

 the salivary glands and pancreas of higher animals, and 

 which digest starchy food to sugar in preparation for ab- 

 sorption. Both in plants and animals there are enzymes 

 which have the power to change starch, sugar, oil or fat, and 

 proteins in various ways to fit them for absorption, trans- 

 portation, or storage. 



121. Water as a transporting medium for foods, oxygen, 

 and excretions plays an important part in the life of higher 

 animals and plants, being the basis of the blood in animals and 

 of sap in plants. In each of these liquids, water is the medium 

 in which are dissolved the materials to be transported. The 

 blood of animals contains more oxygen and excretions than 

 the same amount of sap of any plant, because greater activity 

 in animals means that more oxygen is required, and more 

 oxidation means that more excretions are produced. The 

 amount of dissolved food in a given quantity of sap of a plant 

 and blood of an animal might be the same, that is, as much 

 sugar or other substance as can be dissolved in water at a 

 given temperature. 



There is one striking difference between the transporting 

 liquids in animals and in plants; namely, that in animals the 

 liquids (blood and lymph) move in a circuit from the heart 

 around through the tissues of the body and thence back to 

 the heart. Hence there is a circulation of the blood in most 

 of the animals which have blood, and there is an organ (heart) 

 for keeping up the circulation. In plants, on the contrary, 

 there is no circulation. Water from the soil moves up in the 

 wood-tubes of plants, and water holding in solution foods 

 and other matters moves down in the inner bark (sieve-tubes) 

 of plants ; but there is no direct connection between the two 

 sets of tubes, and hence no possibility of a complete circuit. 

 However, the important point is that both plants and ani- 

 mals use water for transporting foods, oxygen, and excretions ; 



