176 



HUTCHINSON'S POPULAR BOTANY 



that push their aerial roots down the trunks of trees in the forest hang them 

 quite free when above the water, only allowing them to branch out when 

 they reach the surface. In the first case moisture is obtained from the rain 

 and the dew as they trickle down the little channels in the bark, while in 

 the other a reservoir of water is below, and the plant seems to know it." 



It is the tip of the root just in advance of the growing point that appears 

 to possess the intelligence. It seems to know when a stone blocks its pro- 

 gress that it is no use trying to get through. It turns aside from the obstacle 

 and goes round it, but persists in pursuing its original direction in spite of 

 this detour. Darwin compared the root-tip to a brain. 



An extremely curious instance of the motility of young roots is furnished 

 by an Indian species of Loranthus, nearly related to the Mistletoe (Viscum 

 album), and, like it, a parasite on trees. The fruit contains bird-lime a 

 peculiar viscous, tenacious, and elastic substance and when the berry 



loosens from the plant, it 

 sticks to whatever it falls 

 upon. The seed is em- 

 bedded in the viscid pulp, 

 and germination com- 

 mences in the following 

 manner. " The radicle," 

 says Mr. N. E. Brown, 

 "at first grows out, and 

 when it has grown to 

 about an inch in length, 

 it develops upon its ex- 

 tremity a flattened disc ; 

 the radicle then curves 

 about until the disc is 

 applied to any object that 

 is near at hand. If the 

 spot upon which the disc 

 has fastened is suitable, 

 the germination continues, 

 and no locomotion takes 

 place ; but if the spot 

 should not be a favourable 

 one, the germinating em- 

 bryo has the power of 

 changing its position. 

 This is accomplished by 

 the adhesive radicle rais- 



ing the geed and advancing 



; f f Q nr>fV,aT. tmnf /-.i- fr> 

 lt( to anOtnei Spot, Or, tO 



FIG. 220.-RHUBABB (Rheum rhaponticum). 



[E.step. 



Well known as a kitchen-garden plant whose leaf-stalks are used in tarts, 

 etc. It is a native of Siberia, whence it was introduced about 350 years ago. 



