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 Kceberliniaspinosa to 6.6 mm. in Cereus 

 gig-antcvs. 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 arc 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 

 decree 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 exceedingly 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 Cactace;e and Conifene, as well as 

 Elodea, Chara, etc., arc 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 old 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 J-'/odea can be exposed to 

 light more intense than 60 times concentrated sunlight without injury. If 

 a comparison of the life conditions of Jilodea with that of the desert forms 

 is permissible, we might expect the chloroplastids of Cereus 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 ehloro- 



*Compare distribution of chlorophyll in Parkinsonia, p. 23. 



