CAUSES OF THE MOTION OF JUICES. 371 



for a week or more, the rootlet will grow down into the 

 water and the stem will reach the cork. As often as the 

 position of the seedling is reversed, so often the root and 

 stem will reverse the direction of their growth. This ex- 

 periment being carried on in total darkness, save during 

 the short intervals necessary for observation, the directive 

 tendency is shown to be independent of the action of light. 



Causes of Directive Power. — The direction of growth 



in plants appears to be for the most part the consequence 

 of the action either of gravitation simply, as in those 

 parts which extend directly downwards, or of internal 

 tension overcoming gravitation, as in the parts which grow 

 vertically upwards, or lastly of a combination (resultant) 

 of the two forces in the parts which extend in the inter- 

 mediate directions. 



The parts of a plant, whether the individual cells or ag- 

 gregates of cells, are either in a state of tension greater or 

 less and varying at different times, or they are entirely 

 j)assive. 



In general, tension prevails in most parts of common 

 plants ; the full-formed roots, stems, leaves, etc., maintain 

 their relative positions against opposing forces, and when 

 bent, recover themselves with more or less elasticity and 

 completeness. 



There are, however, jjoints where tension is absent or 

 equally exerted towards all sides, and is hence unable to 

 give direction to growth. This may be the case where 

 the tissue, consisting exclusively of newly-formed and im- 

 mature cells, having delicate walls, possesses but little 

 firmness, but is j)lastic like a semifluid substance. In such 

 a condition of growth the cells follow the stress of gravi- 

 tation or of any external force that may be accidentally 

 applied. 



Influence of Gravitation. — ^Most young roots are in 



this passive condition near the tips in the region where 

 Digitized by Microsoft® 



