262 PART III. VEGETABLE PHYSIOLOGY. 



average time it takes in each case to complete a revolution, and note the utility 

 of these movements in bringing the sensitive petioles into contact with objects 

 suitable for support. Observe also the change that takes place in the thickness 

 and strength of the petioles after they have clasped a supporting object. Watch 

 carefully also to see whether, in either of these leaf climbers, any circumnutating 

 movements of the leaves themselves are discernible. 



7. Observe in the common Ivy that, when grown in a window, its leaves 

 turn toward the light, while the young shoots bend away from it, and that the 

 numerous secondary roots thrown out along the side of the stem all point 

 directly away from the light. Compare with these the rootlets of the Trumpet 

 Creeper and Poison Rhus. The apheliotropic tendrils of the various species of 

 Ampelopsis are also very interesting subjects of study. , 



The common basket plant, Saxifraga sarmentosa, produces pendent sto- 

 lons, or runners, which, when the plant is grown in a window, turn away 

 from the light. 



Another instructive experiment is to obtain a glass of water from a pond 

 which contains numerous microscopic green organisms, such as Desmids, the 

 spores of Algse, etc., a condition which is generally indicated by the greenish 

 tinge which the organisms communicate to the water. Place the glass in a 

 window for a few hours and then examine. On the side exposed to the light a 

 green incrustation will be observed, showing that the organisms have moved 

 toward this part of the enclosure and there accumulated. 



8. Try the following experiment in hydrotropism. Cause some peas to 

 germinate in moist sand. After the protruding radicles have a length of about 

 half an inch, remove the peas from the sand and fasten them to an apparatus 

 constructed as follows : Take a piece of wire gauze or mosquito-netting, and 

 having arranged a quantity of Sphagnum moss, fasten the gauze or netting 

 around it in such a manner as to form a cylinder about an inch in diameter and 

 about six or eight inches long. In a row along one side of this attach the ger- 

 minating seeds in such a manner that when the cylinder is inclined at an angle 

 of about 45 , with the seeds on the lower, sloping surface, the radicles will point 

 nearly perpendicularly downward. Having secured the cylinder in the position 

 thus indicated, keep it in a moderately dark, and not too dry room, and keep 

 the moss saturated with water for twelve hours or more, and then examine. If 

 the experiment has been carefully conducted, it will be observed that the 

 radicles have deviated from their normal downward course toward the moist 

 surface of the cylinder. 



9. Sleep-movements may readily be studied in the leaves of any of our 

 species of Sorrel (Oxalis stricta, O. violacea or O. acetocella), in those of the 

 common Locust (Robinia pseudacacia), and in those of the Sweet Clover (Meli- 

 lotus alba). Observe the movements carefully and make drawings illustrating 

 the diurnal and nocturnal positions of the leaves in each of these species. 



The opening and closing movements of the following flowers may also 

 readily be observed : The Morning-glory, the Dandelion, the Sow-Thistle, the 

 Evening-Primrose, the Four-o'clock, and the white Water-lily. Note the time 

 of opening and closing in each case when the weather is clear, and observe 

 whether cloudy weather in any way modifies the movements. 



