PARKINSONIA. 



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PARKINSONIA ACULEATA, P. MICROPHYLLA, AND P. TORREYANA. 



(Plate 4 and figs. lo and ii.) 



Parkinsonias are small trees which occur in this vicinity in habitats that 

 nsually are distinct. P. aculeata is found native on the lower slopes of the 



Coyote Mountains, about 

 50 miles west of Tucson, 

 but is cultivated in the 

 .gardens of the city. P. 

 niicropJiylla occurs on 

 Tu manioc Hill and on 

 the low, dry hills in the 

 western portion of the 

 Laboratory domain. P. 

 torreyana is growing- in 

 the wash at the western 

 base of Tumamoc Hill. 

 The three species are 

 green in all parts, from 

 which the common 

 name, pah verde, is de- 

 rived. P. aadeata and 

 torreyana carry more 

 leaf -surface, or at least 

 larger leaves, than mi- 

 crophylla, in which they 

 arc extremely small. In 

 each species portions or 

 all of the leaves fall away 

 during unfavorable sea- 

 sons . The general struc- 

 tural relations of the 

 stem do not need special 

 notice; they will be ap- 

 parent from the discus- 

 FiG. 10— Parkhisoma microphylla: A, segment of stem sion of the chlorophyll 

 3 mm. in diameter; ^, transverse section of woody cylinder apparatus, 

 to show presence of chlorophyll in wood parenchyma ^^ ' 



adjoining a duct and in the medidlary rays. As in all the Young branches, i.e., 

 other sketches the stippling indicates the presence of ^^ose 1 cm. or less 

 ^ ^^'^^ ^ ' in diameter, are abun- 



dantly supplied with chlorophyll, which is distributed in characteristic fash- 

 ion from epidermis to pith. In general terms this distribution may be 

 defined as follows: It occurs in the cortex as three separate bands concen- 

 trically placed in the medullary rays of cortex and of wood, in certain of 

 the wood parenchyma, and in the pith. This is the maximum chlorophyll 



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