D1V. II 



PHYSIOLOGY 



293 



If mechanical effects lead to wounding the result may be the 

 phenomena of healing (p. 164) or restitution (p. 282). 



5. Chemical Influences. The presence of the necessary nutrient 

 substances in sufficient quantity and the absence of poisonous 

 substances are formal conditions for growth. While it is known that 

 particular, essential, nutrient materials are not replaceable by an 

 excess of others, some substances may be of special importance in 

 particular processes. Since elongation is essentially due to the 

 introduction of water, the signifi- 

 cance of the water supply to a 

 growing plant is obvious. Growth 

 often ceases when there is not 

 sufficient water in the soil. Even 

 a diminution in the humidity of 

 the air may arrest growth by 

 increasing transpiration. Some 

 plants, however, can store water, 

 and are therefore more inde- 

 pendent of its direct absorption. 

 They grow at the expense of the 

 stored water, and can often with- 

 d raw the water from older portions 

 so that these wither while growth 

 goes on at the apex, as is shown 

 by potatoes sprouting in a dark 

 cellar. Plants in damp situations 

 are usually larger than those 

 grown in dry places, and in fact 

 may differ from them in their 



whole habit and mode of growth. FlG - 262. -Young plant of Lupine, the main root of 



P , -i which has become curved. The lateral roots 



A local excess of water in the have arisen on the convex faces of the curves. 



plant, such as may be brought (After NOLL.) . 



about by arresting transpiration 



by a coating of paraffin oil, may lead to various departures from the 



normal structure ( 596 ). 



A striking stimulus -effect results from permanent contact with 

 liquid water in such plants as can endure this. This is doubtless the 

 result of the combined effect of a number of factors and not simply to 

 the material effect of the water. Thus both the arrest of transpiration 

 and the change in the illumination are of importance. 



Amphibious plants, that is such as are capable of living both upon land and 

 in water, often assume in water an entirely different form from that which they 

 possess in air. This variation of form is particularly manifested in the leaves, 

 which, so long as they grow in water, are frequently linear and sessile or finely 

 dissected, while in the air their leaf-blades are much broader and provided with 

 petioles (cf. Fig. 128). The leaf-stalks and internodes also often exhibit a very 



