LAND AND WATER PLANTS. 239 



A COMPAEISON OF LAND AND WATEE PLANTS. 



By Professor GEORGE JAMES PEIRCE, Ph.D., 



LKLAND STANFORD JR. UNIVERSITY. 



THE aquatic origin of all living things is now a generally accepted 

 conception. The arguments in its favor are : ( 1 ) morphological, 

 based on comparative studies of the vegetative and reproductive parts; 

 (2) biological, based on observation of the habits of plants and ani- 

 mals, especially at the breeding season; (3) paleontological, based on 

 the now known fossil remains of formerly living organisms; (4) 

 physiological, based on the absolute dependence of all living things on 

 water. These last arguments appeal to me more strongly than any 

 others. When we realize that all food, all the materials of which the 

 body is constructed, and all the substances which its cells use, can 

 enter the cells only in solution in water, we see at once how indis- 

 pensable water is. When we realize besides that the form and size of 

 the cells, and therefore of the body, depend upon the pressures within 

 the cells which are due to the presence of aqueous solutions therein, 

 we see how necessary water is in another way. Upon the tension of 

 the cells depends the mechanical force which they, the tissues, and the 

 organism, can exert. The absolute dependence of all living things 

 upon water is one of the two most important characters which they 

 possess. The amount of water which different cells, organs and organ- 

 isms use varies greatly, but they all require some water. The ease 

 with which different organisms, organs and cells obtain water also 

 varies, though not necessarily in a degree corresponding with the 

 amounts used. If we compare the conditions under which water and 

 land plants live, we shall see some reasons for the differences in the 

 structure and habits of these two classes. 



The constantly submersed aquatic, whether fresh or salt water, is 

 buoyed up with a very considerable force. A solid mass of plant tissue 

 from which all air and water had been pressed would be buoyed up in 

 water by a force from seven to eight hundred times as great as would 

 be exerted if it were in the air. This is in accordance with Archimedes' 

 well-known law in physics — a body in a fluid is buoyed up with a force 

 equal to the weight of the volume of fluid which it displaces. Any 

 part of a land plant, therefore, which rises into the air is supported with 

 say only one seven hundredth of the force which supports the sub- 

 mersed aquatic. This difference is met by the land plant in two ways. 

 It develops tissues which mechanically support it, which carry that part 



