72 THE FLOWERING PLANT. 
be observed, under the microscope, to move bodily round the 
cells in the direction of their length. Streaming protoplasmic 
currents can also be observed in the cells of many hairs. An 
instructive and common instance is found in the stinging hairs 
of nettle. If one of these is carefully removed from a young 
leaf, and examined under a microscope, it will be found to con- 
sist of a unicellular hair produced into a hollow brittle spike 
terminated by a minute knob. The base of the hair is embedded 
in a dome-shaped emergence. The inner part of the protoplasm 
Fig. 29.—Pinnate Leaf of Scarlet Runner in the position of ‘‘Sleep” [after Sachs]. a. 
The large motile organ at base of grooved leaf-stalk and its continuation d, d; ), c. 
small motile organs of the leaflets, e, e, e. Stipules are seen at base of leaf-stalk, 
also an axillary bud; stipels at the origins of the leaflets. 
within the hair is broken up into a network of strands by means 
of vacuoles. Currents taking various directions can be observed 
in the strands. It may be noted in passing that the stinging 
property is due to the presence of formic acid in the cell-sap. 
The tapering end of the hair readily perforates the skin, when 
it breaks off and the poison flows into the wound. Movements 
on a larger scale are also exhibited by leaves, of which the best 
known example is the sensitive plant. The leaves of wood- 
sorrel, scarlet runner, and many other plants perform what are 
known as “sleep”? movements, by which the leaflets sink down 
