CHAPTER IV. — PRACTICAL EXERCISES. 261 



over the edge of the block, and be free to move. To each of the radicles, near 

 its end, fasten by means of a minute drop of shellac varnish a delicate straight 

 filament of glass about two inches long. The filament is readily made by soft- 

 ening a thin glass tube or rod in a gas-flame, and then rapidly drawing it out. 

 The distal end should be blackened so that it may readily be seen. Now 

 replace the seeds, with the block to which they are attached, under the bell-jar 

 next one side, so that any movements of the blackened ends of the filaments 

 may readily be observed through the side of the jar. By noting from time to 

 time, with dots of ink on the side of the jar, the position of the filaments, very 

 considerable movements will be observed. 



Besides the circumnutating movements that may thus be studied, it will be 

 seen after a time that the radicle has bent downward. This is a geotropic 

 movement. 



Experiment in a similar manner with germinating acorns and peas, and 

 record the results. 



4. Place a few of each of the above-mentioned seeds on the surface of 

 moist sand, cover them with a bell-jar and keep them moderately warm. The 

 radicles will, after a time, be observed to emerge from the seed-coats, bend 

 downward and penetrate the sand. A few hours after this has taken place, 

 carefully remove the germinating seeds, and it will be observed that numerous 

 sand grains adhere to the radicles above their tips. Many of these still adhere 

 to the radicles after rinsing them with water. On examining them with a mag- 

 nifier, it will be found that they are firmly cemented to the root-hairs that have 

 formed in great abundance above the apex of the radicle. So secure is their 

 attachment that they can scarcely be dislodged without destroying the hairs 

 themselves. 



5. Plants of the Hop and Morning-glory are easily obtained, and in them 

 the twining movements are readily studied. Begin the study in each case with 

 the young stem just emerging from the soil. Observe that at first it grows erect, 

 or nearly so, but after two or three internodes are formed, the upper one bends 

 slightly to one side and begins its revolving movements. As this internode 

 elongates and others are developed above it, the rapidity of movements for a 

 time increases, and the circle described widens. But for each plant there is a 

 limit, for when the internode attains its full growth, it becomes rigid and usu- 

 ally erect. Note in each case the average time it takes for an internode to 

 complete a revolution, and how many revolutions are completed before its 

 movements cease. It would be interesting to observe also how the movements 

 are affected by different temperatures, and how by the presence or absence of 

 light, or by light of different intensities. 



To observe how these movements serve the plant for climbing, thrust one 

 end of a thin stick into the ground beside the plant, and the stem will soon be 

 seen to coil spirally around it. Observe that the Hop twines with the sun, or 

 in the same direction as the hands of a watch move, while that of a Morning- 

 glory twines in the opposite direction. 



6. The common garden Tropeolum and the native Clematis Virginiana, 

 so common in our woods, both afford good examples of leaf-climbers. Observe 

 with care the movements of the young internodes in each of these species, the 



