GENERAL DISCUSSION. 33 



GENERAL DISCUSSION AND RESULTS. 

 MORPHOLOGY OP THE CHLOROPHYLL-BEARING TISSUES. 



Although different in details, the general arrangement of the chlorophyll 

 apparatus in the stems of desert perennials has in many respects a close 

 similarity, for which reason the type arrangement can be presented by 

 describing an ideal stem. 



The ideal branch will have a diameter of 5 mm., with the following 

 structural divisions: (l) an epidermis with a relatively heavy cuticle; 

 (2) a hypodermal tissue about three cells in thickness; (3) an outer cortical 

 chlorophyll band, which is somewhat wider than the hypoderm and which 

 lies immediately beneath it; (4) a hard-bast ring; (5) the inner portion of 

 the cortex, in which are the distal ends of the medullary rays, perhaps 

 secondary hard bast and undifferentiatcd ground-tissue; (6) the woody 

 cylinder and the pith. Medullary rays are prominent and wood parenchyma 

 is present in considerable amount. 



Such being the general structural relations of the stem, the chlorophyll 

 is distributed as follows: The leading chlorophyll-bearing tissue is the 

 subepidermal band, and this is true not only because it is the most exten- 

 sive of the stem, but also, as will be shown below, because it retains chloro- 

 phyll the longest of any of the tissues. Moreover, it is rarely changed into 

 any other kind of tissue, but persists as chlorenchyma until, as a general 

 thing, it is cut off by the formation of cork. 



The medullary rays, both of the wood and of the cortex, contain chloro- 

 phyll. The distal ends of the medullary rays extend into the cortex and 

 between the groups of hard bast and abut on the inner face of the chloro- 

 phyll band, by which circumstance the chlorenchyma of the stem is for the 

 most part bound into one continuous system. In addition to the chlorophyll 

 band and the distal ends of the medullary rays, there is in the cortex another 

 kind of chlorenchyma, namely, the ground-tissue, that lies immediately 

 within the hard-bast groups. In the woody cylinder, in addition to the 

 medullary rays, the wood parenchyma also may contain chlorophyll. This 

 is true especially of those cells that are placed near the ducts. Lastly, the 

 pith may be chlorophyllaceous. Generally speaking, therefore, with the 

 exception of certain embryonic tissues in the cortex, all of the parenchyma 

 of the stem may contain chlorophyll. 



As the stem increases in diameter, certain changes in these tissues, as a 

 whole, and in those which are chlorophyll-bearing, are seen to take place, 

 some of which should be noted. These may be outlined as (l) the gradual 

 disappearance of the chlorophyll and (2) as modifications in the topography 

 of the chlorophyll apparatus, as above sketched. 



In the usual condition there is a certain order in the recession of chloro- 

 phyll in the stem. This is as follows: It disappears first from the pith; 



