EESPIEATION OF PLANTS. 15o 



and physical causes having been adduced in explanation. By some 

 they are considered as connected with the nourishment of the cell, 

 the presence of the nucleus, and the process of cytogenesis. Certain 

 authors have referred the phenomena to endosmOse, dependent on 

 varying density in the cell-contents, while electrical agency has been 

 called into requisition by others. In Ohara the chlorophyll granules 

 lining the walls of the cells have been supposed to exercise a galvanic 

 action upon the sap, and thus. give rise to the motion. 



Dr. Pettigrew, from experiments by which he succeeded in inducing 

 similar movements artificially, concludes that the ultimate causes are 

 mainly physical, of which absorption, resulting in endosmose and 

 exosmose, and evaporation, are the chief; and that the phenomena 

 are influenced by the general circulation. He says, " while the cells 

 in the root of the plant inaugurate the general circulation, the general 

 circulation in its turn influences the intracellular circulation. This 

 follows, because when a current of fluid travels up the one side of a 

 thin porous cell-wall, and another and opposite current travels down 

 the other or opposite side, a certain proportion of the currents pass 

 obliquely through the ceU-wall, and cause the fluid contents of the 

 cell to gyrate or move in a circle. The cell-contents are made to 

 gyrate, even in the absence of opposing currents outside the ceU, 

 if endosmotic and exosmotic currents are induced within it ; or if 

 evaporation or capillarity be made to act at certain points." 



3. — Respiration of Plan,ts. 



The changes which are produced in the atmosphere by living 

 plants have been included under the title of Vegetable Respiration. 

 The experiments of Priestley, in 1771, show that plants when ex- 

 posed to light in an atmosphere containing a considerable proportion 

 of carbonic acid, purify the air by removing carbon and producing 

 oxygen. Air in which animals had died was thus rendered again fit 

 for breathing. Percival confirmed those observations. Scheele made 

 a series of experiments with nitrogen in place of carbonic acid, and 

 he found that plants did not purify an atmosphere composed of 

 nitrogen alone. The foul air then, in his experiments, differed com- 

 pletely from that in Priestley's experiments, and hence the difierence 

 of results. Ingenhouz and Senebier performed numerous experiments, 

 which proved that during the day plants gave out oxygen gas, whUe 

 during darkness this process was suspended. The former has shown 

 that the green portions of all vegetables, irrespective of their specific 

 properties, are equally available for such operations ; that it is from 

 the under surface of the matured leaves that oxygen is chiefly given 

 off; and that in plants placed in shade the action of the leaves 

 does, not prevent deterioration of the air. Saussure stated that 



