MOVEMENTS OF IRRITATION. 509 



194. Anisotropy. 



The direction which plant structures take in growth and finally 

 maintain, by no means depends on accidental circumstances, but 

 in a definite way stands in a causal relation to a series of internal 

 and external growth determinants. Hence it is even in many 

 cases possible to set forth, at least to a certain extent, the causes 

 which lead a plant structure under given conditions to take up one 

 and no other direction of growth, and a number of these cases may 

 here find special mention. 



If we cultivate seedlings of Phaseolus in the manner described 

 in 172 in a zinc box, behind a sheet of glass, it is easy to make out 

 that the lateral roots of the first order make a definite angle with 

 the main root, which we term the geotropic limiting angle. Our 

 culture has been placed in the dark, and when a number of lateral 

 roots have developed we indicate the direction of their apices by 

 making ink-lines on the outside of the sheet of glass. We now 

 expose our apparatus to diffuse daylight, keeping the temperature 

 constant. The tips of the lateral roots undergo a considerable 

 change in their direction of growth, as can be unmistakably seen 

 even at the end of twenty-four hours. Even in the light of course 

 the roots do not grow vertically downwards, but still the geotropic 

 limiting angle of the newly formed parts of the roots is consider- 

 ably less than that of the parts which originated in darkness. 

 Light, therefore, is able to influence in a definite way the geotropic 

 behaviour of roots. 



If in spring we examine the soil in the neighbourhood of flower- 

 ing plants of Adoxa Moschatellina, we find that it is traversed by 

 numerous ivory-white rhizomes of the plant. They run horizon- 

 tally in the soil, and the question arises, what conditions bring 

 about the plagiotropism of the organs. We cut off the ends of 

 some of the rhizome branches to a length of several centimetres, 

 stick them with their lower ends in moist soil contained in a flower- 

 pot so that their apices are directed upwards, and put them in a 

 dark place under a bell-glass. After a few days (in my experi- 

 ments after two or three days) the growing parts of the rhizome 

 tips are directed horizontally, and the growth curvature, as 

 follows from our experiment and others, is due exclusively to the 

 action of gravity. Stress must, however, be laid on the fact that 

 the geotropism of the rhizome of Adoxa does not cause it to 

 assume a vertical but a horizontal direction. 1 



