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HANDBOOK OF PHYSIOLOGY. 



ing the nucleus and occupying the cell cavity; its interstices are filled 

 with fluid. ID young vegetable cells such a distinction does not 

 exist; a finely granular protoplasm occupies the whole cell-cavity 

 (fig. 17, A). 



Another striking difference is the frequent presence of a large quan- 

 tity of intercellular substance in animal tissues, while in vegetables it is 

 comparatively rare, the requisite consistency being given to their tissues 

 by the tough cellulose walls, often thickened by deposits of lignin. 

 As an example of the manner in which this end is attained in animal 

 tissues, may be mentioned the deposition of lime salts in a matrix of 

 intercellular substance which occurs in the formation of bone. 



(2.) As regards the respective functions of animal and vegetable cells, 

 one of the most important differences consists in the power which vege- 

 table cells possess of being able to build up new complicated nitrogenous 



Fig. 17. (A.) Young vegetable cells, showing cell-cavity entirely filled with granular protoplasm 

 inclosing a large oval nucleus, with one or more nucleoli. (B.) Older cells from same plant, show- 

 ing distinct cellulose-wall and vacuolation of protoplasm. 



and non -nitrogenous bodies out of very simple chemical substances ob- 

 tained from the air and from the soil. They obtain from the air, oxy- 

 gen, carbonic anhydride, and water, as well as traces of ammonia gas; 

 and from the soil they obtain water, ammonium salts, nitrates, sulphates, 

 and phosphates, and such bases as potassium, calcium, magnesium, so- 

 dium, iron, and others. The majority of plants are able to work up 

 these elementary compounds into other and more complicated bodies. 

 This they are able to do in consequence of their containing a certain 

 coloring matter called chlorophyll, the presence of which is the cause of 

 the green hue of plants. In all plants which contain chlorophyll two 

 processes are constantly going on when they are exposed to light : one, 

 which is called true respiration and is a process common to animal and 

 vegetable cells alike, consists in the taking of the oxygen from the at- 

 mosphere and the giving out of carbon dioxide; the other, which is 

 peculiar apparently to bodies containing chlorophyll, consists in the 

 taking in of carbon dioxide and the giving out of oxygen. It seems that 

 the chlorophyll is capable of decomposing the carbon dioxide gas and 

 of fixing the carbon in the structures in the form of some new com- 



