100 BOTANY. 



timetre or two fasten the seeds in such a way (under a bell- jar) that 

 the radicles point directly upwards. Observe that the roots soon 

 begin bending towards the earth. 



174. Electricity. — While plants exhibit electrical condi- 

 tions in common with other material objects, they seem at 

 present to possess no physiological significance. Every 

 chemical change in the cell probably produces some dis- 

 turbance of its electrical conditions and of those of its 

 neighboring cells. So, too, the considerable amount of 

 evaporation of water from leaves and other aerial parts 

 probably produces electrical disturbances. Various ob- 

 servers have noticed weak electrical currents between differ- 

 ent tissues upon making transverse sections of stems or 

 leaves. None of these appear to be of any importance 

 physiologically, at least as now understood. Strong elec- 

 trical currents, especially when interrupted, quickly dis- 

 organize the protoplasm; weak currents retard or arrest 

 protoplasmic movements, and very weak currents produce 

 no perceptible effect. 



175. Humidity of the Air. — The walls of living plant-cells 

 are usually permeable to water, and when exposed to rela- 

 tively dry air they lose a portion of their watery contents 

 by evaporation. In many-celled plants this loss is renaired 

 by the absorption of water from contiguous cells not so ex- 

 posed, and the latter in turn repair their loss by absorption 

 from the surrounding moisture (water or moist earth). 

 The condition of the atmosphere may thus set up many dis- 

 turbances in the plant. 



176. Since evaporation of water takes place so generally 

 in our common plants, it has been sometimes supposed to 

 be one of the necessary activities of the plant, and is spoken 

 of as Transpiration. It is, however, a purely physical 



