CAUSES OF THE MOTION OF JUICES. 871 
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 
passive. 
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, points 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 plastic 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 
