26 PLANTS AND MAN 



nitrates, phosphates and sulphates the glucose can be converted 

 into proteins and the latter in turn into new desmid protoplasm. 

 As this cytoplasmic activity continues some of the oxygen released 

 by photosynthesis is used up by respiration; but an active cell, on 

 sunny days, liberates more oxygen than can be used by respira- 

 tion with the resulting passage of oxygen out of the desmid cell 

 into the water of the environment. At night, however, photo- 

 synthetic activity ceases so that oxygen has to pass into the cell to 

 continue the respiration process. This balance between the oxy- 

 gen released by photosynthesis and that used up in respiration 

 is quite variable, depending upon the rate of food consumption 

 and food formation. This same variable balance holds true for 

 the carbon dioxide; ordinarily, in the daytime, the amounts of 

 this gas given off by respiration are used up at once within the 

 cell by the photosynthetic process, so that actually, in the hours 

 of sunlight, respiration does not result in the giving off of carbon 

 dioxide to the environment. At night, or during periods of 

 photosynthetic inactivity, the carbon dioxide is given off to the 

 surroundings just as in animal respiration. Such a balance be- 

 tween the oxygen and carbon dioxide taken in and given off holds 

 true for all green plants, aquatic or terrestrial. 



Of the other fundamental activities essential to plant exist- 

 ence, those of growth and protection are as simple as those con- 

 cerned with metabolism. The size of the cell is limited, so that 

 when this maximum volume is attained, excess protoplasm for- 

 mation results in a splitting of the cell to form two new desmids; 

 growth and asexual reproduction in this case being closely re- 

 lated processes. Protection against dessication is unnecessary for 

 aquatic plants since they are continually bathed in a watery 

 environment. Protection against physical injury is well taken 

 care of by the rigid cell wall which, in many species, develops 

 spines and sharp projections which undoubtedly discourage 

 hungry aquatic animals. This life of a desmid plant is typical of 

 all green members of the plant kingdom, with the fundamental 

 demands of existence uncomplicated by numerous cells, tissues 

 and organs. It is an existence characterized by an ability to solve 

 all of life's pressing problems in an elemental and simple 

 fashion. 



