WHAT IS A PLANT? 155 



atmosphere and heat, must have some internal system, governed 

 by some intrinsic force, by which the prepared food could be con- 

 veyed to all parts of the body. Thus came about a circulatory 

 apparatus, culminating in the higher forms in an apparatus con- 

 sisting of heart, arteries, veins, capillaries, and lymphatics. Sq 

 argued the great naturalist ; but he admitted the absence of such 

 a system among some lower forms, thus practically resigning its 

 diagnostic value. Of course, circulation goes on in plants, but 

 relatively to the digestive process, and from a morphological point 

 of view, it presents no similarity to animal circulation, although it 

 performs an analogous function. 



Circulatory organs, as such, are absent in Protozoa, Ccelen- 

 TERATA, and PoRiFERA. Still, a kind of circulation is kept up in 

 the animalcules by the cilia referred to before, and by the 

 contractile vacuoles, which probably also subserve the function 

 of respiration ; in the Sponges, moreover, we have a system of 

 canals, with "pores" for water to enter by, and " oscula," or Httle 

 mouths, for its escape. We are justified, however, in saying that 

 these animals are devoid of circulatory organs, and Cuvier's 

 second test thereby becomes invalid. 



VIII. — Presenxe of Nitrogen. Cuvier held that because 

 animals needed muscles to move with, and nerves to guide move- 

 ment and receive impressions, therefore the chemical constitution 

 was more complex in animals than in plants. This was effected 

 by the presence of nitrogen, which he set down as his third dis- 

 tinction. Chemical research has made rapid strides since 1828, 

 and we know now that nitrogen is always present in plants, and 

 quite as important to them as to animals, seeing that the proto- 

 plasm, or " sarcode " — i.e., the living substance of plants and 

 animals, the "physical basis of life," as Huxley rightly calls it — is 

 essentially and absolutely identical in both kinds of living beings, 

 whether high up or low down in the scale, whether in the germ or 

 in the adult. Its presence, therefore, as a means of diagnosis 

 between the two kingdoms, is hopelessly useless, and must vanish. 



IX. — Gas-Exchange ; Function of Respiration. If we fill 

 a wide-mouthed bottle with water, place some green leaves in it, 

 and expose the whole to sunlight, bubbles of oxygen gas will soon 



