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, may be, 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 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 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 Ce/tis 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. 
Species. Gonoen: Gia neces 
ad mm. mm, 
Celtis pallida ........cc ee 8 4 
Cereus giganteus............c0:00 (7) 6.6 
Condalia spathulaca 1.5 0.75 
Covillea tridentata............0+- 3 1.5 
Fouquieria splendens............. 30 0.9 
Keeberlinia spinosa...............- 3.5 0.38 
Krameria CaneSCens.......ce-e0++ 2.5 1.25 
Olneya tesota.........ccecereeeeereee 535 B75 
Parkinsonia microphylla......... II 5.5 
torreyana.... 22.5 6 
Prosopis velutina........ i 3.3 1.65 
Zizyphus EMOryi....-....---seeeeeees 8 4 
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- 
