38 



TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



plastids are subject to an unknown but high degree of insolation; to which 

 the innermost ones are subject to an unknown but exceedingly small degree. 

 Thus there is experienced at the same moment a very wide range in the 

 intensity of the light stimulus. What the effect on the morphology of the 

 chlorenchyma, especially, of this light stress in such a plant as Cereus, in 

 which the chlorophyll-bearing tissues endure throughout the life of the 

 plant, perhaps unchanged, maybe, has not been inquired into, but probably 

 it is a very important factor to be reckoned with and one that must be taken 

 into account in studies on this general subject. 



The relation of the deeply seated chloroplastids in the stem of such plants 

 as Cerens or Parkinsonia to air is very different from such relation in leaves, 

 where the character of the structure insures abundant aeration. To a 

 relatively long distance from the source of supply of oxygen and carbon 

 dioxide and small intercellular spaces of the typical xerophyte, is added 

 immobility of stems, so that gaseous exchange between the external atmos- 

 phere and that inside the plant, as well as between different portions of the 

 plant, is not aided by various bendings and movements characteristic of 

 leaves. This may result in a condition in which unusually small amounts 

 of air reach the deeper tissues, so for this reason photosynthesis is precluded. 

 Indeed, the manner of recession of the chlorophyll from the stem suggests 

 that poor aeration rather than the lack of sufficient light may be an impor- 

 tant factor in limiting the depth at which chlorophyll may be functional. 

 It will be recalled that in Celt is pallida, as well as in other forms, the wood 

 parenchyma which surrounds or is most closely related to the large ducts 

 retains chlorophyll after it has disappeared from other portions of the woody 

 cylinder as far removed from the surface or even considerably nearer the 



surface. 



Penetration of the chlorophyll. 



Pfeffer states (loc. cit., p. 329) that enough carbon dioxide may be taken 

 up by the roots, when transpiration is active, to prevent the more deeply 

 seated chloroplastids at the base of the stem from losing the power of assim- 



