214 'carbon-fixation,' 'assimilation' 



during the dull days of winter, the manufacture of carbon com- 

 pounds is usually slow, and is often insufficient to supply the 

 proper needs of plants. Similar partial starvation due to want 

 of light occurs among thickly-planted crops and in the inner 

 boughs of trees bearing an excess of leaves, and in all cases of 

 over-crowded plants. With an increased intensity of light, 

 ' carbon-fixation ' increases proportionally up to a maximum, 

 which for many plants is not attained until they are exposed 

 to direct sunlight. 



Certain shade-loving plants, however, need a moderate 

 intensity of light for proper nutrition ; exposure to intense 

 light retards or altogether suspends their activity in this respect, 

 and at the same time acts injuriously upon their chloroplasts 

 and other protoplasmic cell-contents. 



In the majority of plants, the epidermal cells are free from 

 chloroplasts, and the cell-contents of this tissue no doubt screen 

 the chloroplasts of the deeper-lying tissues from the deleterious 

 action of too brilliant light. Moreover, the chloroplasts are 

 moved into more advantageous positions within the cells, when 

 the intensity of the light falling upon the leaves becomes too 

 great. 



The red, orange and yellow rays present in sunlight are most 

 effective in promoting ' carbon-fixation,' the purple and violet 

 rays having very little effect upon the process. 



In many plants ' carbon-fixation ' goes on to a slight extent 

 at one or two degrees above freezing-point : with increasing 

 temperature the process increases in activity up to about 20° or 

 25° C, beyond which temperatures it decreases until at about 

 56° C. it ceases altogether with the death of the plant. 



Ex. 119. — Place some shoots of Potamogeton, Elodea canadensis, mare's tail 

 (Hippuris) or mint in a beaker full of well water. Slide a glass funnel into 

 the beaker as indicated in Fig. 88, and over the end of the funnel place a test- 

 tube full of water. Expose the whole to bright daylight, and notice that 

 bubbles of gas rise from the leaves of the plants and collect at in the test-tube. 



After a few c.c. of gas have been collected, remove the test-tube, and 



