68 PHYSIOLOGY. 



141. Fungi cannot form starch. If we should extend our 

 experiments to the fungi, which lack the green color so charac- 

 teristic of the majority of plants, we should find that photosyn- 

 thesis does not take place even though the plants are exposed 

 to direct sunlight. These plants cannot then form starch, but 

 obtain carbohydrates for food from other sources. 



142. Photosynthesis cannot take place in etiolated plants. 

 Moreover photosynthesis is usually confined to the green plants, 

 and if by any means one of the ordinary green plants loses its 

 green color this process cannot take place in that plant, even 

 when brought into the sunlight, until the green color has ap- 

 peared under the influence of light. 



This may be very easily demonstrated by growing seedlings 

 of the bean, squash, corn, pea, etc. (pine seedlings are green even 

 when grown in the dark), in a dark room, or in a dark receiver 

 of some kind which will shut out the rays of light. The room 

 or receiver must be quite dark. As the seedlings are " coming 

 up," and as long as they remain in the dark chamber, they will 

 present some other color than green; usually they are somewhat 

 yellowed. Such plants are said to be etiolated. If they are 

 brought into the sunlight now for a few hours and then tested 

 for the presence of starch the result will be negative. But if the 

 plant is left in the light, in a few days the leaves begin to take 

 on a green color, and then we find that carbon dioxide assimila- 

 tion begins. 



143. Chlorophyll and chloroplasts. The green substance in 

 plants is then one of the important factors in this complicated 

 process of forming starch. This green substance is chlorophyll, 

 and it usually occurs in definite bodies, the chlorophyll bodies, 

 or chloroplasts. 



The material for new growth of plants grown in the dark is derived from 

 the seed. Plants grown in the dark consist largely of water and protoplasm, 

 the walls being very thin. 



144. Form of the chlorophyll bodies. Chlorophyll bodies 

 vary in form in some different plants, especially in some of the 



