SUMMARY OF RESULTS. 41 



SUMMARY. 



The leading- results of this study may be summarized as follows: 



1. Young- stems of desert perennials contain chloro]3hyll in most of the 

 parenchyma, both of cortex and of woody cylinder. The most important 

 chlorophyll -bearing tissue appears in transverse sections of the stem as a 

 band in the outer part of the cortex. 



2. The epidermis of branches of Parkiiisoiiia 1 cm. in diameter may 

 contain chlorophyll. 



3. Chlorophyll is present in the i)hell(xlcrm of the following- species: 

 Cc/tis pallida, Condalia spat/iulaca, Olncya tcsota. 



4. There is no chlorophyll in the woody c>'linder of Aster sf)i)iosits or 

 Baccharis cincvvi . 



5. The woody cylinder in young- stems of Ephedra antisyphilitica and of 

 Olneya tesota do not contain chlorophyll; in older stems the woody cylinder 

 of both is chlorophyllaceous. 



6. The chlorophyll band in the stems of Cereus, Fouquicria, I\)aiiicria, 

 Parkiiisonia, and probably also in Zizyphus, persists throug-hout the life of 

 the member bearing- it. In most plants it. is ultimately cut off throug-h 

 the formation of cork. 



7. As reg-ards foliar habits the plants studied may be classified into two 

 g-roups, which, however, are not always well marked. In one class leaves 

 are either rudimentary or wanting-; in the other, leaves are present at least 

 during- the favorable seasons, /. <?., when the water-supply is adequate to 

 their needs. 



8. The differences in leaf-covering- are accompanied by fairly consistent 

 morpholog-ical differences, as follows: The plants with reduced leaf -surface, 

 or with no leaves, have palisade chlorenchyma in the cortex; the chloro- 

 phyll band, at least in young- stems, lies near the surface; the tissues exte- 

 rior to the band in young- and generally in old leaves do not exhibit pro- 

 tective devices. Plants with a more pronounced leaf-surface, on the other 

 hand, have a spong-y chlorenchyma in the cortex; it is usually more deeply 

 placed; and the exterior tissue usually has some protective arrangements, 

 as pigmented cells or a hairy covering. 



9. The greatest depth at which functional chlorophyll was found ranged 

 from 0.38 mm. in I\ivhcrUiiia spiiiosa to 6.6 mm. in Cereiis giganteus. This 

 is from 0.5 to 165 times deeper than the greatest depth of chlorophyll in 

 ordinary leaves. 



10. The depth of penetration is probably limited by the air-supply rather 

 than the supply of light. 



11. The chlorophyll band of the stem constitutes practically the sole 

 engine for carbon assimilation in Aster spinosus, Baccharis emoryi, Cere7is 

 giganteus, Kcvberlinia spinosa, Krameria canescois, and the most important 



