CHAPTER III. PRACTICAL EXERCISES. 245 



ones, and the latter may be regarded as parasitic on the former. 

 Flowers and ripening fruits also, like true parasites, continually 

 absorb oxygen and give off carbon-dioxide, and they as effectu- 

 ally drain the vegetative portion of the plant of its resources, as 

 the Mistletoe does the Elm on which it grows, or the giant 

 Rafflesia does its host-plant. 



Practical Exercises. 



1. Take a branch three or four inches long cut squarely off from a thriftily 

 growing plant of Anacharis Canadensis, an aquatic very common in slow 

 streams and ponds, without permitting the rest of the plant to dry, wipe the 

 water off from the cut end and apply a little shellac varnish, permitting this to 

 get thoroughly dry. With a needle-point now puncture a minute hole in the 

 varnished end and then immerse the plant, this end upward, in a narrow jar 

 containing pond or spring water. Expose it now to the light, and if the experi- 

 ment has been properly conducted, small bubbles will be seen to issue in rather 

 rapid succession from the aperture in the varnished end of the stem. These 

 bubbles have been proven to consist chiefly of oxygen gas. 



Transfer the jar to a dark room, and after it has remained there for a few 

 minutes examine. It will be found that no gas is now given off. The evolu- 

 tion of bubbles will again take place, however, on restoring it to the influence 

 of the sunlight. 



Lower the temperature of the water by packing ice around the jar, and the 

 evolution of gas again ceases, but it begins again when the temperature is per- 

 mitted to rise sufficiently.' 



Now pour off the pond or spring water and substitute for it some freshly 

 distilled water, or what will answer equally well, some spring water that has 

 been boiled for some time to expel the dissolved carbon-dioxide ; then expose 

 the plant to strong sunlight as before. No gas will now be evolved ; but it will 

 immediately begin again if carbon -dioxide be introduced into the water. This 

 may be done by simply shaking the water in a deep glass jar in the bottom of 

 which a candle has been permitted to burn for a few minutes, or enough may 

 introduced by merely blowing the breath into the water by means of a tube. 



2. On a cold winter's day remove a branch of Hickory, Butternut or Maple 

 and bring it into a warm room ; the change of temperature, causing expansion 

 of gases in its tissues, soon produces bleeding from the cut surface. Take the 

 branch out into the cold again and the bleeding stops. The experiment may 

 usually be repeated several times with the same branch. What is the cause of 

 the flow of sap from the Maple tree when tapped in early spring ? 



3. Obtain four small flower-pots, and, having filled them with good black 

 earth, plant in one of them a few grains of barley, in another a few of wheat,in 

 another some of Indian corn, and in the fourth some seeds from a pumpkin. 

 Keep them all moist, though not too wet, and let the room in which they are 

 placed be maintained at a constant temperature of about 45 F. At the end of 

 a week examine ; the barley and wheat grains will probably be found in various 



