WHAT LIMITS PHOTOSYNTHESIS 41 



of chlorophyll, plants will not be healthy without it, for 

 they will not be green. Except for pine seedlings, ordi- 

 nary plants will not form chlorophyll in darkness. This 

 fact is utilized in the blanching of celery and asparagus, 

 as well as being a matter of common observation. 



What Limits Photosynthesis. — Certain definite 

 factors can be recognized as limiting and controlling 

 photosynthesis. These are: 



(1) The amount of chlorophyll exposed to the light, 

 — This will vary, not only with the quantity of pigment 

 actually contained in the chromatophores but with the 

 number and the exposure of the chlorophyll-containing 

 cells in leaves and stems. Plants with scanty leafage 

 offer less chlorophyll exposed to light than those with 

 more abundant foliage. 



(2) The amount of light that becomes effective. — 

 Plants or plant-parts may be shaded out of existence. 

 This is clear in forest and chaparral, for the foliage — 

 and often the lower branches — thus die or never develop 

 in the shadow of branches and foliage above them. Com- 

 paratively few species of plants are able to thrive in the 

 shadow of other plants or of opaque objects. Shaded 

 plants usually make up for deficient light by greater ex- 

 posure of chlorophyll, either by forming chlorophyll in 

 the epidermis covering the leaves, or by larger and 

 broader leaves. 



(3) The quality of light. — The light reaching the 

 forest floor may be as unsuitable in quality as it is in- 

 sufficient in ^quantity, for it may lack the particular rays 

 used in photosynthesis by reason of their being already 

 absorbed by the foliage above. 



(4) The amount of warmth. — Differences in tempera- 

 ture are found to affect photosynthesis in nature more 

 than any other single factor. Photosynthesis will not 

 take place in most plants at temperatures close to the 

 freezing point. The rate of photosynthesis rises more 

 rapidly than the temperature, however, up to a certain 



