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 



IlG. 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; b, 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 



