128 



BOTANY OF THE LIVING PLANT 



happens even to leaden pipes when defective. The roots make their 

 way through the soil to the source of water. That moisture exercises 

 a directive influence on the growth of the root may be shown by 

 germinating seedlings on the outer rim of a porous flower-pot kept 

 moist from within. The first roots, growing over the edge of the pot, 

 follow its outer sloping surface, instead of growing vertically down- 

 wards. The hydrotropic influence of the moist surface is here stronger 

 than that of gravity. It is this hydrotropic curvature of roots which 

 explains how they grow towards, invade, and choke porous or leaky 

 pipes. 



Another familiar fact illustrates the sensitiveness of living plants 

 to a further stimulus, that of the free access to atmospheric 



air. When plants are grown 

 in porous earthenware pots, 

 especially if they are left too 

 long in small pots without re- 

 potting, the roots form a dense 

 mat close to the inner surface 

 of the pot. Such plants are de- 

 scribed as "pot-bound. 7 ' This 

 condition is due to the fact that 

 the roots grow towards a source 

 of free oxygen. The pot is 

 porous, and allows of gaseous 

 interchange with the air outside. 

 Such a response is styled aero- 

 tropism, and roots curving to- 

 wards the source are positively aerotropic. The opposite is, however, 

 seen in pollen- tubes (see Chapter XVI.). If pollen-grains be cultivated 

 on a slide in a suitable sugar solution, under a cover-glass, the resulting 

 pollen-tube curves away from the margin, where they would be subject 

 to free aeration, thus showing negative aerotropism (Fig. 89). In this 

 case the reacting body is a simple non-septate tube. Thus an un- 

 divided protoplast may be irritable, receiving and reacting to external 

 stimulus, and modifying its growth accordingly. Such examples of 

 irritability as these described, either in simple or complex parts, are 

 all referable to the living protoplast, and involve movement in growing 

 parts. 



But on the other hand, external stimuli, if their action be continued 

 during growth till the rigidity of the mature state is attained, may 

 leave a permanent, and often a profound effect upon the final form. 



FIG, 89. 



Pollen-grains germinated in a nutritive medium 

 under a cover glass, of which the margin is shown. 

 The tubes curve away from the margin, that is, 

 away from the supply of oxygen. (After Molisch.) 



