436 PHYSIOLOGY OF GROWTH. 



are seen to be traversed in every direction by plasmodial strands. 

 The trophotropism of the fungus is thus established. 



About the middle of a plasmodium, spread out on a moist sub- 

 stratum, we place a small crystal of common salt. The part of 

 the fungus in direct contact with the salt becomes brown and 

 dies, while those portions of it which are not killed retract from 

 the salt, so that gaps are formed in the plasmodium, which 

 however may close up again if the gradually dissolving salt 

 becomes uniformly enough distributed in the moist substratum. 

 Sodium chloride therefore exerts not an attractive, but a repul- 

 sive action on the plasmodium. 1 



That the extraordinary sensitiveness of the plasmodium to the 

 influences above mentioned, as also to others, is of biological 

 importance to their delicate organisation will be clear from what 

 has gone before, and we need not here consider the matter 

 further. 



1 See Stahl, Botan. Zcituncj, 1884, No. 10. Here also will be found in- 

 structions for further experiments, together with citations of literature. 



II. GEOTROPIC, HELIOTROPIC, AND HYDROTROPIC 



NUTATIONS, AND SOME OTHER PHENOMENA OF 



IRRITABILITY. 



172. The Geotropic Behaviour of Hoots. 



The roots, especially the main roots of plants, strive to grow 

 vertically downwards, a phenomenon which is due to the positive 

 geotropism of the organs. To prove that roots are positively 

 geotropic, we conduct the following experiment: Seeds of Pisum, 

 Vicia Faba, or Phaseolus are soaked for twenty-four hours in 

 spring water, and then placed in large flower-pots or wooden 

 boxes containing moist sawdust. The sawdust must be very 

 loose and uniformly moistened throughout in order to ensure the 

 normal development of the seedlings. The seeds of Vicia F;il);i 

 are laid in the sawdust with their micropyles downwards, so that 

 the emerging main root need make no curvature. The seeds of 

 Phaseolus we lay horizontally ; the main root then makes a 

 right angle with the long axis of the seed on emergence. We 



