LIMITS OF CHLOROPHYLL DISTRIBUTION. 37 
PENETRATION OF THE CHLOROPHYLL. 
The greatest depth at which chlorophyll was found beneath the surface 
of the plants varied from 0.38 mm. in A wberlinia spinosa to 6.6 mm. in Cereus 
giganteus. In ordinary leaves chlorophyll occurs from 0.04 mm. to 0.35 mm. 
from the surface. As contrasted with the depth of chlorophyll in leaves, 
that in the stem is, therefore, from about 9 to 165, or 0.5 to 18.8 times more 
distant. In these desert plants, of a consequence, there are very unusual 
conditions to which the chlorophyll of the stems may be subjected and 
under which photosynthesis may be carried on. The most deeply placed 
chlorophyll probably has the minimum amount of light, or the minimum 
degree of aeration, or the least amounts both of air and of light. Adequate 
water-supply as well as suitable temperature, more surely the latter,* are 
presupposed to exist. 
As is well known, chloroplastids of certain plants may exercise their 
function of carbon assimilation under cxceedingly feeble illumination. 
Pfeffer (Physiology of Plants, Eng. ed., vol. 1, p. 340) states that photo- 
synthesis may occur at an illumination 1/600000 the intensity of sunlight. 
It is not surprising, in view of this, in a region where light is so intense as 
in the desert, that we find chlorophyll over 0.5 cm. beneath the surface 
(in extreme instances probably much deeper than this). 
The light tonus probably plays an important role, as indicated by the range 
of the position of chlorophyll in the stem. This condition is well known 
in plants inhabiting more moist regions. Chloroplasts of many plants 
become pale and discolored after a few days in darkness. The paling of 
grass and of low herbaceous plants in weak light which obtains during a 
long wet season are familiar examples of the dependence of the chloro- 
plastids of such forms on a constant and considerable supply of light. On 
the other hand, plants belonging to the Cactaceze and Coniferz, as well as 
Elodea, Chara, etc., are more resistant and may remain green for a month 
or more in darkness. But the fact that at a depth of 6.6 mm. the chloro- 
plastids of Cereus are green in o/d stems is indication not of survival but of 
their being functional at the moment. It is not known what the maximum 
light stimulus may be that the chloroplastids of Cereus may endure without 
injury, but Pfeffer states that chloroplastids of Avodea can be exposed to 
light more intense than 60 times concentrated sunlight without injury. If 
a comparison of the life conditions of Z/odea with that of the desert forms 
is permissible, we might expect the chloroplastids of Cerews and of other 
desert types to be exceedingly resistant to light. 
The considerable extent of the chlorophyll apparatus in the desert plants 
emphasizes another condition which is probably not present in leaves, or if 
so, to a limited extent only, namely, what may be termed the light stress 
which the protoplast of the desert plant experiences. The outer chloro- 
*Compare distribution of chlorophyll in Parkinsonia, p. 23. 
