1 86 



PLANT LIFE. 



introduce variations in tlie rate of growtli, wliich obscure, but 

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

 iS6. ) 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 [leriod is a[)parent 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. 

 jAmphibious plants, that is, those 

 which are capable of growing either 

 on land or in water, often show this 



Fig. 1S7. — a siioot of water in a Striking way. When grown sub- 

 crowfoot {Rami j^i: 1(1 us i 1 i n r i 



rt,/«ni'/7«). The lower leaves merged, the leavcs are usually finely 



have developed under water ,. ■ , t t ., , , ■ ,- 



and are branched uito many dlVldcd, Whlle the Sailie lea\eS, it 

 narrow divisions; the two , , . , , , 



upper leaves have developed allowed to QCVelop in the air, llave 

 in air and at maturity lloat 1 i 1 t 



on the surface of the water, uroacl blades Scarcely more tlian lobed 



About half natural size. — 



After Frank. ( hg. I 87). 



265. Mechanical pressures or strains also e.xert an in- 

 fluence upon the rate and motie 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 tliose exerted bv wind 

 or weight, promote the develojiment of mechanical tissues. 

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

 enduring a pull of 500 gni. for five da)'s, broke only at i 600 

 gm. After ^\\t days more under a strain of 1200 gm. they 

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



266. Variations in rate. — There are not onl)- variations 

 in growth in the course of each day throughout the growing 



