i6 



EXPERIMENTAL PLANT PHYSIOLOGY. 



the chimneys and note that a bright deposit of copper has formed 

 on the nails. Pour some granulated zinc into each cylinder. In 

 24 hours take out the undissolved zinc, and filter the solution in 

 both jars, to obtain the copper precipitate. Allow the precipi- 

 tate to remain on the filter-paper and dry. Weigh. It will be 

 found that a much smaller quantity is obtained from the solu- 

 tion in the apparatus containing the nails. The action of the 

 iron nails in withdrawing the copper from the solution inside the 

 lamp-chimney, thus causing an additional amount to be taken up 

 from the outside, will show the manner in which a plant exercises 

 a " selective power " of absorption. 



14. Turgor. — When a living cell, composed of protoplasm 

 enclosing the cell-sap and surrounded by 

 the wall, is placed in contact with vifater it 

 absorbs the water in such quantity that the 

 wall is stretched, while on the other hand 

 the wall tends to contract by its own elas- 

 ticity. Thus a cell-tension is set up which 

 is denoted turgor (Fig. 11). The cells 

 composing many of the tissues do not 

 absorb water, while others take up large 

 quantities and expand in consequence. 

 If now a tissue which absorbs much water 



^. , „ ,„ is attached to another which remains pas- 



Diagram of cell. (Har- ^ 



tig.) a, c, wall; l>, svve, a s\.ra.\n, ov tissue-tension, w\\\ be set 



protoplasm ; d, nucle- 

 us ; ^, cell- sap. up. These tissue-tensions give rigidity 



to herbaceous plants. The wilting of plants is accompanied 

 by loss of turgor, and consequent decrease of the tissue- 

 tensions. 



EXPERIMENT 20. 



TURGOR IN AN ARTIFICIAL CEIL. 



Cover one end of an open glass cylinder 10 cm. in length (a 

 large tube will suffice) with membrane, fill with a sugar solution, 

 close the other end in the same manner and place in a vessel 



