186 



PLANT LIFE. 



introduce variations in the rate of growth, which obscure, but 

 do not counteract, the retarding influence of light. (See fig. 

 186. ) This daily period is so impressed upon the constitu- 

 tion of the plant that it maintains it for a considerable time 

 even when kept in complete darkness. Stems of sunflower, 

 after two weeks in complete darkness, still showed distinctly 

 the daily period. A similar daily period is apparent in 

 the tension of tissues which depends 

 on growth. 



264. Moisture and oxygen. — The 

 amount of moisture and oxygen pres- 

 ent in the medium surrounding a 

 plant profoundly affects its form. 

 Amphibious plants, that is, those 

 which are capable of growing either 

 on land or in water, often show this 



Fig. 187. — a shoot of water in a striking way. When grown sub- 

 crowfoot {Ranunculus , , . 

 aquatuis). The lower leaves merged, the leaves are usually finely 



have developed under water . , 



and are branched into many divided, while the 



narrow divisions; the two 



upper leaves have developed allowed to develop 



in air and at maturity float 



on the surface of the water, broad blades Scarcely 



About half natural size. — 



After Frank. (fig- 187). 



same leaves, if 

 in the air, have 

 more than lobed 



265. Mechanical pressures or strains also exert an in- 

 fluence upon the rate and mode of growth. Compression of 

 tissues retards their growth; strains accelerate it. Thus, 

 stems enclosed in plaster casts or ligatured grow more slowly 

 in thickness. Tensile strains, such as those exerted by wind 

 or weight, promote the development of mechanical tissues. 

 Petioles, which would break under a strain of 700 gm., after 

 enduring a pull of 500 gm. for five days, broke only at 1600 

 gm. Alter five days more under a strain of 1200 gm. they 

 could not be broken with less than a weight of 6500 gm. 



266. Variations in rate. — There are not only variations 

 in growth in the course of each day throughout the growing 



