420 Minnesota Plant Life. 



remoxed iVoni the water tliat l)iit)}s them up. Thus, in the 

 jiipewort. which, from the 1)()ttnm of a lake, lifts a thread-like 

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

 not be thus developed under ordinary terrestrial conditions. 



A g^reat 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, iiorne 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 heav\' 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 l)lade, or Hat jwrtion, 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, 

 gi\ing the strength of an arch to the lower surfaces. The net- 

 w^ork of the large leaf is stronger than that of the small leaf, just 

 as a large umbrella must h^ve a stronger frame than a child's 

 parasol. If leaves or fruits are ])roduced close to the ground 

 they ma}' become larger, while remaining more delicate in struc- 

 ture, than if they were jiroduced 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 jirime recpii- 

 site in ])lant architecture, the organism i^ subjected to xarious 

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

 properly constructed areas. .\s exam])les of such agencies, there 

 might be mentioned currents of air. which if \iolent are often 

 known to damage the bodies of plants; ciu'rents of water, to 

 which plants grcnving in rapid streams are particularly exi)osed ; 

 the action of wax'cs. to which the various surf ])lants nuist adapt 

 themsehes; and the pressure of soil, air and water. b\ which the 

 dilTcrent parts of plants are constantly alTecied. In erect ter- 

 restrial N'egelation elasticity is to a certain degree a re(|uisite of 

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

 nnsiipporlc-d, and exposed to the inlluence of wind or surf. 



