PKACTICAL STUDY. 305 



814. Practical study of iissimilatiou. Before examining the 

 remaining conditions of assimilation, a simple experiment is liere 

 described b3' which the reader can study in their proper relations 

 all the essential conditions of the process, and thus obtain a 

 clearer idea of the means by which the activit}' of assimilation 

 is measured and the indispensable character of the conditions 

 established. 



Fill a flve-inoh test-tube, provided with a foot, with fresh drinlc- 

 ing water. In this place a sprig of one of the following water 

 plants, — Anacharis Canadensis, Mj-riophyllum spicatum, M. 

 verticillatum, or any leaf}' Myriophyllum (in fact, any small- 

 leaved water plant with rather crowded foliage). This sprig 

 should be prepared as follows : Cut the stem squarely off, four 

 inches or so from the tip, dr^- the cut surface quickly with 

 blotting-paper, then cover the end of the stem with a quickly 

 drying varnish, for instance asphalt-varnish (see 115), and let 

 it dry perfectly, keeping the rest of the stem if possible moist by 

 means of a wet cloth. When the varnish is drj-, puncture it by 

 a needle, and immerse the stem in the water in the test-tube, 

 keeping the varnished larger end uppermost. If the submerged 

 plant be now exposed to the strong rays of the sun, bubbles of 

 oxygen gas will begin to pass off at an even and rapid rate, but 

 not too fast to be easily counted. If the simple apparatus has 

 begun to give off a regular succession of small bubbles, the fol- 

 lowing experiments can be at once conducted. 



(1) Substitute for the fresh water some which has been boiled 

 a few minutes before, and then allowed to completely cool : by 

 the boiling, all the carbonic acid has been expelled. If the plant 

 is immersed in this water and exposed to the sun's raj's, no bub- 

 bles will be evolved ; there is no carbonic acid within reach of 

 the plant for the assimilative process. But, 



(2) If breath from the lungs be passed by means of a slender 

 glass tube through the water, a part of the carbonic acid exhaled 

 from the lungs will be dissolved in it, and with this supply of 

 the gas the plant begins the work of assimilation immediatch'. 



(3) If the light be shut off, the evolution of bubbles will pres- 

 ently cease, being resumed soon after light again has access to 

 the plant. 



(4) If glass of different colors be interposed in the path of the 

 sun's rays, it will be shortly seen that orange light differs from 

 violet light in its effects upon the rate of the evolution of the 

 bubbles. 



(5) Place around the base of the test-tube a few fragments of 



20 



