168 ACADEMIC BOTANY. 



carbon in the blood with the oxygen in the air ; and they 

 inhale oxygen, which is vitally necessary to their support. 

 Plants consume this animal poison and convert it into food 

 for both plants and animals, whilst they give off quanti- 

 ties of the oxygen, which animals need. Plants therefore 

 maintain the equilibrium of life : this is why parks and 

 gardens with herbage and trees are so important to the 

 health of cities. 



412. The Saprolegnia — one of the lowest cryptogams — is a curious 

 exception to this law of plant life. It absorbs oxygen, and is often 

 parasitic on flies, which are thus destroyed for want of their proper 

 food. It attaclis young live fishes in their breeding-houses, and after 

 killing them it flourishes on their remains as a Saprophyte (Gr. sapros, 

 putrid). Saprolegnia ferax is easily procured.' Fill a glass with water 

 from a garden-tub, throw a dead fly in it, and the Saprolegnia will 

 develop in a few days. The body of the fly will be covered with 

 hyaline (nearly transparent) threads, radiating around it in the form 

 of a zone. Under the microscope these threads are seen to be continu- 

 ous, simple, or slightly branched. They have a motion similar to that 

 of the hairs of phanerogams. They rapidly produce spores either by 

 fission or fertilization, — the same plant often exhibiting both forms of 

 reproduction. 



LESSON XXXIII. 



FORCES : PHYSICAL, CHEMICAL, VITAL, VOLUNTAKY. 



413. Physical and chemical forces. 414. Vital force ; Cyclosis. 

 415. Special movements. 416. Sensitiveness. 417. Cunning; Sport. 

 418. Voluntary motion. 419. Sleep. 



413. Physical force is prominent in absorption and circu- 

 lation ; chemical force in digestion. These we have con- 

 sidered. 



414. Vital Force. Ch/closis. — But in the midst of the 

 operations of these two forces we see still another power at 

 work, which leads us back to the threshold of life, with its 

 impenetrable secret, as jealously guarded here as in the cell 

 of the Eed Snow. If we take a many-celled hair from the 



