398 



the Conferva tribes produce reproductive spores, which swim 

 actively through the water till they meet with a convenient 

 locality, to which they attach themselves, and then germinate 

 and grow like plants. 



8. The nutritive functions and apparatus in animals and 

 plants present characteristic differences. The presence of 

 a digestive sac, or stomach, in animals, and its absence from 

 plants, is frequently stated as one of their most exact dis- 

 tinctions ; but if we call the interior of the membranous 

 sac, which forms the entire animal in the case of the 

 hydatid, its stomach, the cavity of a vegetable cell seems 

 to have an equal claim to this distinction. "It is impos- 

 sible," says Dr. Lindley, that the whole interior of a 

 living and independent cell is not a stomach." With this 

 ground of distinction, to which kingdom would the Poriphera 

 or Sponges belong ? 



9. The chemical action of animals upon the air is likewise 

 directly opposed to that of plants. Animals, by the process 

 of respiration, remove oxygen from the air, and substitute 

 an equal volume of carbonic acid. Plants decompose car- 

 bonic acid, and restore the pure oxygen to the air. This 

 is a very general distinction, but not universal. Amongst 

 plants, the class of Fungi form a striking exception, for they 

 resemble animals in their action on the air; on the other 

 hand, the Frustulia salina, the curious zoophyte before 

 mentioned, constantly evolves pure oxygen ; and it was the 

 observation of this peculiarity which led to the chemical 

 analysis of its tissue. 



10. Lastly, there is a very general distinction as to the 

 nature of the food required by animals and by vegetables. 

 Vegetables convert inorganic matter into the substance of 

 their own tissues ; animals can derive support only from 

 organized substances, or organic principles. But this 

 general rule is not entirely without exception : water. 



