PENETRATION OF THE CHLOROPLYLL. 39 
ilating carbon dioxide when exposed to light for long periods in an atmo- 
sphere free from this gas. In Cereus, however, the main source of the gas 
is probably through the stomata, although, as in Ce/#is 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 chlorophyll-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, Cereus, Fou- 
guieria, Keberlinia, Krameria, Parkinsonia, and Zizyphus it exists through- 
out the life of the plant; in markedly leafy plants its importance is less, 
perhaps, but still that it is of great moment in their economy can not be 
doubted. It is least important in the evergreen forms, as Ce/tis pallida and 
Condalia spathulaca. 
The chlorophyll 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.; Aranseria, 15 cm.; Olneya, 120 cm.; Proso- 
pis,123cm. 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 Celts, with a 
chlorophyll band 0.025 mm. wide, would have a volume of 0.098 cm. in a 
stem 100 cm. in length; Kewéerlinia, 0.415; Parkinsonia microphylla, 0.477; 
Prosopis, 0.578. The ratios are, approximately, Celtis, 1; Keberlinia, 4.1; 
Parkinsonia, 4.7; Prosopis, 5.7. Of these plants it will be noted that 
Keberlinia 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 O/neya, and with Ce/#is should be classed Covillea 
and Condalia, whose evergreen habit makes the chlorophyll in the stem of 
ess importance in the assimilative processes. , 
