DIRECTIOXS FOR LABORATORY STUDY. 399 



necessary play. On the board, with hole for glass tube as a 

 center, mark an arc of 90 degrees. The radius of the arc should 

 be a multiple of the radius of wheel. Divide arc into half centi- 

 meters. Attach wheat straw to wheel as a pointer. 



To the tip of a growing seedling bean fasten a thread by a slip 

 noose. Pass thread over wheel once and to its free end attach a 

 light weight — just enough to turn wheel and pointer when plant 

 is lifted. Set pointer at o and at intervals read the multiplied 

 growth. By taking observations at regular intervals determine 

 the rate of growth of stem for a week. What regular variation 

 can you discover ? 



30. To show the necessity of respiration for growth. (^[^[242, 245.) 

 Germinate a number of beans in sawdust. Select eight with 



straight roots about 2 cm. long. Clean and dry the surface 

 slightly by brushing with frayed edges of strips of filter paper, 

 taking care not to expose roots so long that they are injured by 

 dry air. With a very fine sablehair brush and thick Chinese (or 

 waterproof black drawing) ink, mark each root by distinct lines 

 into ten spaces 1 mm. apart, commencing with tip. This can be 

 done most conveniently by pinning the seedling to a strip of soft 

 wood and laying alongside the root a ruler whose graduated edge 

 has been blunted by a plane until it is about 2 mm. thick. 



Pin half the seedlings to a strip of soft wood set into a jar 

 partly filled with wet sawdust, so that the roots will be vertical 

 in damp air. Put the other half into a similar jar and cover them 

 with water recently boiled and cooled. After 24 hours, remeasure 

 and compare total growth. (See also exp. 31.) 



31. To determine the zone of maximum growth in roots and stems. 

 (1 258.) 



A. Observe the four seedlings of exp. 30, whose roots grew in 

 moist air. Which spaces grew most? 



B. Mark several upper internodes of a bean plant in a similar 

 way, but at 5 mm. intervals. After 4S hours observe how many 

 have elongated and which have grown mosti 



32. To show the effect of gravity as a stimulus on roots. (" * 287- 

 290.) 



Arrange the marked root of a seedling bean as in exp. 30, ex- 

 cept that the root is horizontal, and a pin just above the extrem- 

 ity murks its position. After 24 hours observe curvature and 

 which spaces have become curved. Compare with those which 

 have grown most. 



