l62 OUTLINES OP PLANT LIFE. 



even when kept in complete darkness. Stems of sunflower 



after two weeks in complete darkness still showed distinctly 



the daily period. 



223. Moisture and oxygen. — The amount of moisture 



and oxygen present in the medium surrounding a plant pro- 

 foundly affects its form. Amphibious 

 plants, that is, those which are capable 

 of growing either on land or in water, 

 often show this in a striking way. 

 When grown submerged, the leaves 

 are usually finely divided, while the 

 same leaves, if allowed to develop in 

 the air, have broad blades scarcely 

 more than lobed (fig. 122). 



Fig. 122 -a shoot of water 224. Mechanical pressures or 



crowioot (Ranuncuiies 



aquatius). The lower leaves gtrains also cxert an influence upon 



have developed uDder water * 



and are branched into many the rate and mode of grOWth. Com- 

 narrow divisions ; the two 



upper leaves have developed pression of tissues retards their growth ; 



in air and at maturity float ^ o ^ 



on the surface of the water, strains accelerate it. Thus, stems en- 



About half natural size.— 



After Frank. closed 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. After five days more under a strain of 1200 gm. 

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

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

 in growth in the course of each day throughout the growing 

 period, but also minor variations independent, so far as 

 known, of external conditions, which are therefore called 

 spontaneous variations. Irregular variations occur from hour 

 to hour in the course of the day. Regular spontaneous vari- 

 ations, also, occur in various organs, particularly in the ten- 

 drils of climbing plants, and in the leaves of flowers and buds. 

 These regular variations, which affect different sides of flat- 



