28 



BOTANY 



of direction, indicates to the root the whereabouts of the 

 water which it needs. The perception of water aids that 

 of direction and under normal conditions the two co- 

 operate. If, however, there be no water in the soil, the 

 inherited instincts of the plant lead it to penetrate even 

 the driest sand. 



If the plant is in such a position that the two stimu- 

 lations do not co-operate, but are antagonistic to each 

 other, the chief instinct of the plant becomes evident, 

 and it can be shown that its great object is the coming 

 into relationship with water rather 

 than with soil. 



If some seedlings are allowed to 

 grow on a sieve which is covered 

 by a layer of moss they will at the 

 outset put out their roots through 

 the holes of the sieve and grow 

 downwards in a normal way, seek- 

 ing as their inherited instincts tell 

 them the soil which should nor- 

 mally be situated below them. If 

 the sieve is suspended over a pan of water, so that moist 

 air is below the roots, they keep on growing downwards as 

 if growing into earth. But if the conditions be changed 

 after the roots have attained a length of, say, half an 

 inch, the air below them being made very dry by artificial 

 means, while the moss in the sieve is kept well wetted, the 

 roots soon curve upwards and growing in opposition to 

 gravity turn towards the water (Fig. 10). They appear 

 to recognise that their original instinct is deceiving 

 them and that the true habitat for them is for some 

 reason above and not below them. If after they have 

 established this new direction of growth the conditions 

 be again changed and moisture be restored to the air 

 below them while the moss is allowed to dry, another 

 reversal of the direction of growth takes place and 



FIG. 10. Hvdrotropism 

 (After Gibson.) 



