Ch. IV, 9] GEOTROPISM OP STEMS 177 



Seeking a connection between geotropism and the plant's 

 pliysiology, we can find no direct relation, such as shows so 

 clearly with phototropism in the relation between light and 

 photosynthesis. It is true, under the influence of geotro- 

 pism the roots are guided down into the soil, and the shoots 

 up into the air, and therefore to the positions essential to 

 their functions ; but, as one immediately recalls, those parts 

 would take those positions without any geotropism, — for 

 the shoot would be guided upward by phototropism and 

 the roots do^vnward by hydrotropism (an adjustment to 

 moisture later considered). Geotropism, therefore, produces 

 exactly the same results in shoots and roots as would photo- 

 tropism and hj'drotropism. But there is this difference, that 

 the light and moisture are variable, and often (especially the 

 light) absent altogether at times when the plant is in growth 

 and needing guidance, while gravitation is perfectly con- 

 stant at all times. It seems wholly probable, therefore, that 

 this invariabihty of action through all external conditions 

 is a reason why plants employ gravitation as well as light 

 and moisture in guiding the growth of their shoots and 

 roots. Precisely the same principle is illustrated in our 

 human affairs where we regulate our daily actions, our 

 risings, our meal times, and our occupations, by the clock, 

 instead of trusting to the height of the sun, often obscured, 

 or our appetites, often spasmodic. In similar manner the 

 sailor steers by his compass, even when sun or stars are 

 visible. 



It must not, however, be inferred that growing towards 

 gravitation or away from it are inseparable from the nature 

 of a main root and main stem respectively. Thus, some 

 main stems have the transverse or horizontal geotropism, 

 as in ground vines. It is for this reason that Ground Pine, 

 our creeping Ferns, and Solomon's Seal, keep their main 

 stems down upon or just under the surface of the ground, 

 instead of sending them upwards. Moreover, the geotropism 

 of parts can change during life, as familiar in cases where a 



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