PENETRATION OF THE CHLOROPLYLL. 39 



ilating- carbon dioxide when exposed to lig-ht for long- periods in an atmo- 

 sphere free from this gas. In Cerens, however, the main source of the gas 

 is probably through the stomata, although, as in Ce/tis and other plants, 

 the roots may be of importance as organs of aeration as well. 



The table on the preceding page presents the greatest observed depths to 

 which chlorophyll penetrates and remains green in the stems of perennials. 

 Considerable care was exercised in selecting material and the estimates in 

 each instance are probably conservative. 



IMPORTANCE OF CHLOROPHYLL BAND. 



As has been already discussed, the leading chloroj^hyll -bearing tissue in 

 the stem is the subepidermal chlorenchyma, which in this paper has been 

 designated the chlorophyll band. This also is the most enduring chlor- 

 ophyll tissue of the stem. It constitutes practically the entire carbon 

 assimilative apparatus in plants with reduced transpiring surface— a very 

 important part of the apparatus in deciduous plants — as it does the entire 

 apparatus, or nearly so, in the leafless forms. In Baccharis, Cereiis, Fon- 

 qiiieria, Kceberlinia, Krameria, Parkinsonia, and Z/st/^//?/.? it exists throiigh- 

 out the life of the plant; in markedly leafy plants its importance is less, 

 perhaps, but still that it is of gfreat moment in their economy can not be 

 doubted. It is least important in the evergreen forms, as Ce/tis pallida and 

 Condalia spathnlaca. 



The chlorophjdl band has been identified in the following plants at the 

 several distances given from the tip, which are not supposed to represent 

 the maximum distance in any case, but may do so: Celtis, 178 cm.; Con- 

 dalia, 95 cm.; Covillea, 95 cm.; Franseria, 15 cm.; Olneya, 120 cm.; Proso- 

 pis, 123 cm. The relative importance of the band appears more clearly, 

 perhaps, when its volume in several plants is compared. In order to insti- 

 tute the comparison the diameter of stem nearest 1 cm. was taken, and the 

 measurement of the chlorophyll band applied directly to an ideal stem 

 100 cm. in length and 1 cm. in diameter. In this manner Celtis, with a 

 chlorophyll band 0.025 mm. wide, would have a volume of 0.098 cm. in a 

 stem 100 cm. in length; Kceberlinia, 0.415; Parkinsonia microphylla, OA77; 

 Prosopis, 0.578. The ratios are, approximately, Celtis, 1; Kwberlinia, 4.1; 

 Parkinsonia, 4.7; Prosopis, 5.7. Of these plants it will be noted that 

 Kceberlinia and Parkinsonia rely mainly or wholly on the chlorophyll band 

 for their carbon assimilation all of the year and Prosopis a part of the year, 

 while Celtis, which is evergreen, but nevertheless has considerable chloro- 

 phyll in its stem, depends mainly on the extensive leaf-surface. With 

 Prosopis should be classed Olneya; and with Celtis should be classed Covillea 

 and Condalia, whose evergreen habit makes the chloroi:)hyll in the stem of 

 ess importance in the assimilative processes. 



