12 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



The chlorophyll band is made up of cuboid and elongated cells, of which 

 the latter have the long axis placed parallel to the surface of the stem. In 

 addition to the outer chlorophyll band, chlorophyll is found sparingly in 

 the medullary rays both of the cortex and of the wood and in the pith also. 



With increase in diameter the stem exhibits certain changes in its general 

 structure, of which the most important in the present connection are to be 

 found in the cortex. Cork is organized early and is superficial. The 

 phelloderm is in direct contact with the chlorophyll band and probably con- 

 tributes chlorophyll-bearing cells to the latter, although this was not 

 definitely determined. The other changes in the cortex do not affect the 

 distribution of the chlorophyll and may be neglected. 



FIG. 5. Cni>illea tridentata: A, transverse section of stem 5 mm. in diam- 

 eter, showing character of spongy tissue of chlorophyll band; .Z?, transverse 

 section of leaf, showing palisade character of subepidermalchlorenchyma; 

 C, cross-section of stem i mm. in diameter, to show general distribution of 

 chlorophyll. Lettered as in fig. 3. 



The order of disappearance of chlorophyll from the stem was not followed. 

 In a stem 1.5 mm. in diameter chlorophyll was observed in the pith, in the 

 medullary rays of wood and of cortex, and in the chlorophyll band. In a 

 stem 3 mm. in diameter, however, it had practically disappeared from all 

 tissues deeper in the stem than the chlorophyll band; in a stem 9.5 mm. in 

 diameter no traces of chlorophyll were to be detected outside of this band. 

 No chlorophyll was found in another branch 2 cm. in diameter and 145 cm. 

 from the tip, although the primitive chlorophyll band, but without chloro- 

 phyll, was still present. 



