LEAVES 



5 2 3 



green algae 1 and in Cuscuta, where ordinary chloroplasts probably are 

 lacking. 



The influence of external factors upon chlorophyll development. 

 * Light. The general necessity of light for chlorophyll development is 

 shown by the pallor of shoots that develop in the dark (e.g. celery leaves 

 and potato sprouts). However, less light is needed in some cases than in 

 others, as appears from the chlorophyll layer beneath the bark in trees, 

 and from the mosses that grow in relative darkness in the mouths of 

 caves. If sugars and other foods are present in sufficient amount, 

 conifer seedlings, various ferns and liverworts, and many algae become 

 green, even in total darkness, and a number of angiosperm seedlings 

 become green in the dark, if the fruits in which the seeds develop are 

 produced in the light. 2 



On the other hand, various algae have been observed to lose their green color when 

 grown in rich nutrient media in the light, thus becoming physiologically equivalent 

 to fungi. Cuscuta develops chlorophyll in sunlight when grown on a starved host or 

 in water, and redwood shoots without chlorophyll have been observed to behave 

 similarly when detached from the parent tree and placed in water. Perhaps an 

 excess of food, though necessary for chlorophyll production in the dark, may be 

 less necessary or even unfavorable in the light. Carotin and xanthophyll are less 

 dependent upon light than is chlorophyllin, remaining longer when plants are 

 placed in darkness, and appearing sooner when they are placed in the light. 



Temperature and salts. Low temperatures are more detrimental to chlorophyllin 

 than to xanthophyll, the latter appearing first in spring and remaining latest in 

 autumn; the yellowing of evergreens in winter illustrates the same principle. 3 The 

 development of pallid shoots at low temperatures has been observed in Sequoia 

 and in Brassica. Iron salts and nitrates are regarded as favorable for chlorophyll 

 development, whitening due to lack of iron being called chlorosis; common salt 

 impedes chlorophyll development, and perhaps is responsible for the pale color 

 of salt marsh plants, as has been shown to be the case in Salkornia. Plants attacked 

 by parasites often show chlorophyll impairment. 



Albescence. In many plants, especially in certain variegated plants cultivated for 

 ornament (as Abutilon and Caladium), the absence of chlorophyll is not obviously 

 related to external factors. Yellow spots contain plastids colored with xanthophyll, 

 and white spots lack even plastids. Such plants are propagated readily by cuttings 

 and sometimes also by seed. The whitening or albescence of Abutilon is thought 

 to be due to a virus (perhaps an enzym) that is detrimental to chlorophyll de- 



1 The blue-green algae, however, are now believed to possess a single cylindrical chro- 

 matophore in each cell. 



2 Recent studies seem to show that the green pigment of many seeds is not chlorophyll, 

 though it becomes chlorophyll upon exposure to light. 



3 However, a recent investigation shows that chlorophyll sometimes develops in 

 abundance at low temperatures, the minimum temperature recorded being - 8 C. 



