34 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



then from the medullary rays of the wood, beginning in the inmost portions; 

 then from the wood parenchyma; after this, from the medullary rays of the 

 cortex; and, lastly, from the chlorophyll band, or, more usually, the band 

 itself is obliterated. In the exceptional event of chlorophyll in the epidermis 

 of the younger stem, as in Parkinsonia, this may leave the stem before any 

 other tissue is deprived of its chlorophyll. Generally speaking, however, 

 with the exception of the wood parenchyma, the chlorophyll of the stem 

 disappears in a centrifugal fashion. Possibly the exception is due to some 

 favoring condition, as the proximity of better air-supply, or more water, 

 or the light may be condensed by the water-content, so that a portion of the 

 contiguous parenchyma may have better light relations than that in the 

 medullary rays. 



The chlorophyll band remains active until its organic connection with 

 the inner living portions of the cortex is severed by the formation of cork, 

 or until destroyed by pressure occasioned by growth of the stem. In 

 some forms, as Ccrens and Parkinsonia, as a rule, it is never destroyed during 

 the life of the plant, but persists and gives the color characteristic of each. 



SPECIAL STRUCTURAL FEATURES. 



We may turn now from a review of what may be called the general con- 

 dition of the stem as regards the distribution of chlorophyll and take note 

 of phases of the distribution and other characters of the chlorophyll-bearing 

 tissues not shared by all of the plants examined. 



Attention may first be called to a curious modification of the chlorophyll 

 band which frequently accompanies increase in diameter of the stem. This 

 is associated with the formation of cork. The chlorophyll band, properly 

 so-called, is an integral portion of the primary cortex; in old stems, how- 

 ever, the outer part of the band may be morphologically secondary cortex. 

 This circumstance occurs in the following manner: When cork is formed, 

 it is likely to take its origin in cells exterior to the chlorophyll band and 

 very close to it. By the activity of the phellogen periderm is organized 

 without and phelloderm within in the usual fashion and in the cells consti- 

 tuting the phelloderm chlorophyll may be found. Consequently it happens 

 that in older stems of certain plants, as in Celtis pallida, fig. 3, the outer 

 portion of the chlorophyll band, in addition to being of different origin, may 

 be more recently organized than the inner portion. There appears to be 

 a limit to the thickness of the chlorophyll band brought about by this 

 means, however, as the portion of the band which is of secondary origin 

 has never been seen to be more extensive than the primitive part. 



Another point which has to do with the structure of the chlorophyll band 

 relates to the similarity in some cases and the dissimilarity in others of 

 the structure of the band in the stems and the structure of the chlorenchyma 

 of the leaves of the same plant. In all plants studied palisade tissue of 

 some sort was found in the leaves, but in part only of the plants was the 



