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-cells and 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 suj)porting 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 cutioular cells 

 are passively stretched by this inteiior strain. Closer in- 

 vestigation indicates that the matter is somewhat compli- 

 cated. K we strip off the " skin," from a stalk of garden 



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