ASSIMILATION. 29 



Place a few red sea-weeds in pure watei-, and let them remain 

 there for ten hours. What is the color of the water by (1) trans- 

 mitted light? (2) by reflected light? Extract tlie coloring-matter 

 of red sea-weeds by means of alcohol, and compare the alcoholic 

 with the aqueous solution. 



What is the color of an alcoholic extract of tlie bruised tissues 

 of Mouotropa uniflora? 



Etiolation. Keep seedlings in a warm, dark place until they 

 have lost their green color, and then, having removed some of 

 their leaves for immediate examination, place the plants, with the 

 remaining leaves attached, in the light, ilake alcoholic extracts 

 of the blanched leaves and of the gi'een ones, comparing them 

 from all points of view. 



Examine pine seedlings grown in complete darkness, and ascer- 

 tain the nature of the pigment which their green cells contain. 



Carbonic acid and assimilation. Compare at the end of two 

 or three weeks the dry weights of two seedlings grown under the 

 following conditions : Both the seedlings have furnished to them 

 exactly the same kind and amount of soil, and are provided with 

 equal amounts of nutrient solutions at corresponding times ; 

 both are placed under tubulated bell-jars, and have the same 

 amount of moisture in the atmosphere to which tliey are exposed. 

 The seedling in one bell-jar obtains a supply of carbonic acid 

 gas, since there is an opening in the jar through which the en- 

 closed air communicates with that outside containing its normal 

 proportion of carbonic acid. The seedling in the other jar lias 

 no carbonic acid supplied, since a cup which contains potas- 

 sic hydrate deprives the air already in the jar of all its carbonic 

 acid, and an open receptacle, filled with pumice-stone satu- 

 rated with potassic hjdrate, removes all carbonic acid from 

 any air entering the jar. One plant is thus furnished with 

 enough available carbonic acid, the other is in an atmosphere 

 wholly free from it. 



In a modification of the foregoing experiment, supply a known 

 quantity of carbonic acid in aqueous solution to the soil of the 

 second plant, being careful to prevent hj means of a cover of 

 rubber-cloth any escape of the carbonic acid from the soil of the 

 flower-pot into the air of the jar, and after a few days compare 

 the weights of the plants as before. 



Can a water plant derive its carbonic acid from water contain- 

 ing a small amount of sodic bicarbonate in solution ? 



Add to the normal air contained in a freshl}' filled bell-jar, in 

 which a seedling is growing, a known quantity' of pure carbonic 



