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: 
a 
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q 
1896] A RUST AND LEAF CASTING OF PINE LEAVES 437 
has shown, a greater proportion of assimilative tissue than those 
grown in the shade. The cells of the tissue under considera- 
tion are, in almost every case, provided with peculiar infoldings 
(fig. 3), hence the name Avmpallisadenzellen, adopted by Haber- 
landt, Strasburger, and others. 
In longitudinal sections the rows of mesophyll cells are seen 
to be separated by more or less space, which often extends with- 
out interruption from the cortical to the’ fibro-vascular region 
(fig. 4). The infoldings are not easily distinguishable in sec- 
tions of this kind, but are usually seen as apparent partitions, 
and in some cases do not appear at all. 
Within the mesophyll region and completely surrounded by 
the cells composing it occur the two resin ducts. The ducts are 
nearly opposite each other, being placed about equidistant from 
the radius of the leaf. They are cylindrical, completely inclosed 
bya sheath of thick-walled stereomatic cells arranged in a single 
tow, and are lined with thin-walled epithelium (jg. 5). The 
central portion of the leaf is occupied by the fibro-vascular 
region. Separating this from the mesophyll is a closed sheath, 
the endodermis of Oudemans.? This consists of a single row of 
more or less suberized, elongated, thin-walled cells, closely 
joined at the horizontal or oblique ends (fig: % end). The 
walls also contain lignin, as shown by staining with indol. 
Within the endodermis occur the mestome bundles (two in 
tumber), tracheids, and fundamental tissue, the latter consisting 
of thin-walled parenchymatous cells. The orientation of the 
bundles is normal, 7. ¢., the hadrome is directed toward the cen- 
er and the leptome toward the periphery (/igs- 7 and 7). 
LIFE OF NORMAL LEAVES. 
The length of time the leaves live varies according to a num 
ber of conditions, the most important being the age and size of 
ae tree and cold and drouth. On trees 15 to 20° high, grow- 
ng in good soil, leaves may remain on three, four, and even five 
rdam, 
e 
*Ueber den Sitz der Luftwurzeln der Orchideen, Abhandl. d. Acad. Saat 
Math, Phys, Klass. gion, 1861. 
