REMARKABLE PROPERTIES OF ROOTS. 775 



termed hydrotropism. It frequently happens in mountainous districts that after 

 heavy downpours of rain the overflowing streams tear deep furrows in the adjoin- 

 ing steep forest lands, and root up the ground, throwing everything into confusion 

 and depositing below, on the valley-floor a mass of detritus or rubbish. Usually 

 numerous organic bodies, blocks of wood, pieces of turf, leaves, fir-cones, and the 

 like are torn away by these turbulent streams with the stones and sand, and the 

 deposit is therefore studded with nests and strips of humus which owe their oi-igin 

 to the organic fragments mentioned. Seeds of various plants from the neighbour- 

 ing forest are swept into the rubbish heap, and among them those which only 

 flourish well in the damp humus of forest soil. These seeds germinate, and 

 their roots penetrate downwards; many perish at once in the inhospitable soil, 

 but others grow excellently, sending up a vigorous stem and unfolding foliage 

 and flowers. When these well-grown plants are dug up in order to see the 

 relation of their roots to their immediate environment, it at once becomes evident 

 that the roots in their downward progress have curved towards the nests and 

 veins of humus. They exhibit the most wonderful twists and bends, and look 

 as if they had actually been attracted by the humus deposits. Without quite 

 excluding the possibility of a chemical attraction, we must regard the aversion 

 of the roots to dryness as the chief cause of the bending. The masses of humus 

 embedded in sand and rubbish retain moisture like a sponge, and when the 

 adjoining sand-strata have been for long dried up the dark nests and strips still 

 retain their saturated condition. When a root shunning the dryness turns away 

 from the sand, and in continuing its growth comes to a deposit of humus rich 

 in water, it finds there no inducement to continue bending, and so grows straight 

 through the region of the damp layer. When in its further growth it emerges 

 from the ball of humus and enters the dry sand, it of course again bends and 

 curves round the ball of humus, or wheels round in a half circle so as to return to 

 the moist dark clump which is situated like an oasis in the dry desert of sand. 



It is obvious that larger pebbles which cannot be displaced by growing roots 

 must cause a swerving; the root whose tip is in contact with the hard stone 

 bends sideways and evades the insurmountable obstacle lying in its path. A very 

 noticeable bend ensues when the growing root is injured on one side of its tip, 

 or is so fastened to some foreign object that the cells at the place of contact 

 are damaged. It then bends away from the injured or attached side and assumes 

 a divergent course. 



In many cases it might be thought that the roots were not repelled by the 

 unfavourable places in the soil, but were attracted -by the favourable places, and, 

 as already stated, the possibility of an attraction, a, mutual action of the sap of 

 the root and the materials contained in the places in question in the soil, which 

 might find expression in a movement of the growing root-end, is not entirely 

 excluded, although it has not hitherto been demonstrated with certainty. 



The circling, that is, the spiral movement of the growing root, has been explained 

 in various ways. One view was that the cylindrical body of the root may be 



