420 Minnesota Plant Life. 



removed from the water that buoys them up. Thus, in the 

 pipewort, which, from the bottom of a lake, lifts a thread-like 

 stem ten feet or more in length, an organ is formed that could 

 not be thus developed under ordinary terrestrial conditions. 



A great many other adaptations besides these simple ones of 

 length and thickness of shoots might be discussed at this point. 

 For example, it is necessary that seeds or fruits, borne at the 

 tops of trees, should not be of such structure that they would be 

 broken and injured by falling to the ground. Cocoanut seeds, 

 therefore, which are heavy and are produced at a considerable 

 height, have thick, hard shells and are not injured by their fall. 

 A definite relation commonly exists between the stem of a leaf 

 and the blade, or flat portion, so that the leaf is extended in a 

 position such that its leaf-green can do the work of starch-mak- 

 ing under the influence of sunlight. So the stems of large 

 leaves are generally strong and often of a half-cylindrical shape, 

 giving the strength of an arch to the lower surfaces. The net- 

 work of the large leaf is stronger than that of the small leaf, just 

 as a large umbrella must have a stronger frame than a child's 

 parasol. If leaves or fruits are produced close to the ground 

 they may become larger, while remaining more delicate in struc- 

 ture, than if they were produced at a considerable height. In 

 the one instance the fall would be harmful, while in the other 

 the influence of the wind would enter as a factor. 



Mechanical forces. Besides being exposed to the force of 

 gravity acting constantly upon its structure, and consequently 

 rendering due attention to strength of materials a prime requi- 

 site in plant architecture, the organism is subjected to various 

 forces which would tend to demolish it unless it met them with 

 properly constructed areas. As examples of such agencies, there 

 might be mentioned currents of air, which if violent are often 

 known to damage the bodies of plants; currents of water, to 

 which plants growing in rapid streams are particularly exposed ; 

 the action of waves, to which the various surf plants must adapt 

 themselves ; and the pressure of soil, air and water, by which the 

 different parts of plants are constantly affected. In erect ter- 

 restrial vegetation elasticity is to a certain degree a requisite of 

 structure. This is particularly true of such stems as are slender, 

 unsupported, and exposed to the influence of wind or surf. 



