372 HOW CROPS GROW. 
their elongation occurs. The new growth at these points 
simply obeys the attraction of the earth like any other 
limp or yielding mass, and a root made to grow on a 
horizontal plate of glass, for example, is pushed along by 
the expansion of its young cells and the formation of new 
ones until it reaches the edge, when the tip inclines down- 
ward as a wet string would do.- If, however, as many 
times happens, the yielding tissue of new cells is partially 
or entirely enveloped by the more rigid root-cap, the 
downward tendency may be overcome to a corresponding 
degree. In this case the tip keeps more or less closely 
the direction already given to the root, resembling in its 
growth a half melted substance protuded from a tube and 
stiffening as it issues. ‘The passive section of the root is 
translated forward as the root itself extends; the cells that 
to-day yield to the gravitating force, to-morrow become 
so rigid and firmly grown to each other as to resist the 
tendency of this force to coerce them to a vertical, while 
new cells are developed beyond, which conform to the 
gravitating tendency. 
Internal Tension.—In the upward-growing stem the 
different parallel and concentric tissues, viz., the cuticle, 
the cell-tissue of the rind, the wood-calls aad ducts, and 
the pith, exist in a state of unequal tension. 
This is shown by well-known facts. If a hollow, suc- 
culent stem, like that supporting a dandelion blossom, be 
cut lengthwise, the parts curve away from each other, 
thus, )(, and may by a little assistance be rolled together 
in flat coils. The same separation of the halves may be 
observed in any succulent stem, provided it be fresh and 
turgid. It is plain then that the pith-cells of the growing 
stem are compressed by the cuticle; in other words the 
pith-cells are in a state of tension, while the cuticular cells 
are passively stretched by this interior strain. Closer in- 
vestigation indicates that the matter is somewhat compli- 
cated. If we strip off the “skin,” from a stalk of garden 
