38 



TOPOGRAPHY OF CHLOROPHYLL APPARATUS IX DESERT PLANTS. 



plastids are subject to an unknown but hi.u'h degree of insolation; to which 

 the innermost ones are subject to an unknown but exceedingfly 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 lig-ht stress in such a plant as Cereiis, in 

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

 plant, perhaps unchang-ed, 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 i)lants 

 as Cereus 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 oxyg-en and carbon 

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

 immobility of stems, so that g:aseous 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 bending's 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 sugg'ests 

 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 Celtis pallida, as well as in other forms, the wood 

 parenchyma which surrounds or is most closely related to the larg-e ducts 

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

 cj'linder 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 transj^iration is active, to prevent the more deeply 

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



