276 



PRINCIPLES OF PLANT CULTURE 



ing. A twig, an ear of corn or a potato tuber is given 

 to each student and all are encouraged to vie with each 

 other in discovering new points, and in discussing the 

 ^ ^ reasons therefor. This lesson is fre- 



quently repeated during the term. 



Cell structure (12). — The students 

 examine the pulp of a mealy apple and 

 of a potato, and cross-sections of a 

 young bean plant, with simple lenses 

 of rather high magnifying power. If 

 a compound microscope is available, 

 many mounted objects illustrating the 

 cell structure of plants may also be 

 shown. 



Absorption of water by seeds (26). — ■ 

 For the exercises suggested by para- 

 graphs 26 and 27, a means of weighing 

 and of measuring the volume of large 

 seeds, as beans, with some degree of 

 accuracy is needed. The device shown 

 in Fig. 177 answers this purpose, and 

 one can be provided for each pair of 

 students at a moderate cost. It con- 

 sists of a graduated glass cylinder of 

 200 cubic centimeters capacity and a 

 test tube about 6 inches long. For 

 determining the volume, the cylinder is partly filled with 

 water and the height to which the water rises is noted. 

 The seeds are then dropped in and the glass in shaken a 

 little to remove the air bubbles. The height of the 

 water is again noted, when the difference in the two read- 

 ings indicates the volume of the seeds in cubic centimeters. 

 For weighing the empty test tube is placed in the cylinder 



Fig. 177. — Device 

 for weighing and 

 determining the 

 volume of seeds. 



