2l6 THE TEACHING BOTANIST 



and how is the rate affected by dif- 

 ferent external influences ? Answer 

 by Experiment 7. 



(2) What structures in the leaf are con- 

 cerned in this process of transpira- 

 tion ? 



;3) What is the use of transpiration to the 

 plant ? 



Materials. For study of living plant cells, the best object 

 known to me is the stamen-hair of Tradescantia virginica, 

 which is easily obtained in gardens in late spring and summer, 

 but not at other times, unless the plants are cut back in the 

 spring, when they may be made to flower in the late fall ; and 

 if covered at night by a frame and sash, they may be kept in 

 good condition until near December ist. T. pilosa, common 

 in greenhouses, gives hairs less excellent but serviceable. The 

 hairs should be placed in water on a slide under a cover glass. 

 Another classic object for the purpose is Nitella (or Chara), 

 which may be found in streams in summer and kept in aquaria 

 all winter, but they are far less typical than Tradescantia. 

 The latter is particularly valuable because it shows not only 

 a typical cell of the higher plants reduced to about the 

 lowest terms, i.e. nucleus, cytoplasm, vacuoles, and wall, but 

 also shows the cytoplasm in active circulatory movement. 

 Its simple structure makes it very good to begin with, for in 

 studying other cells later the student has little or nothing to 

 unlearn, since others are mostly like it with but additional parts. 

 Good living plant cells may be obtained also from many epi- 

 dermal hairs. Of course, knowledge of the cell should be 

 broadened by observation of mounted as well as other living 



