Tis sties in Certain Monocotyledons. 
61 
EXPLANATION OF FIGURES IN PLATES 
III, IV, AND V. 
Illustrating Messrs. Scott and Brebner’s paper on Secondary Tissues in 
Monocotyledons. 
Figs. 1-3. Development of tracheides. 
Fig. 1. Yucca, sp. Young secondary bundle in tangential section; a developing 
tracheide, with its relatively large nucleus, is shown, x 133. 
Fig. 2. Yucca , sp. Young tracheide isolated by maceration, from a developing 
secondary bundle. The cells to the left are desmogen-cells, for comparison with 
the tracheide, which has only reached about a quarter of the average mature length : 
at a , and perhaps at other points, a short branch is being formed, x 200. 
Fig. 3. Dracaena fragrans , Gawl. Young tracheide and desmogen-cells as in 
last figure. Note that the nucleus is comparatively near one end of the tracheide — 
a common case, x 200. 
Figs. 4-8. Roots of Dracaena. 
Fig. 4. Dracaena fragrans, Gawl. Transverse section of large adventitious root, 
showing secondary thickening. On the right this has taken place entirely outside 
the endodermis ; to the left it has gone on partly inside and partly outside. Por- 
tions of the ruptured endodermis are seen imbedded in the secondary tissues, 
x 17. pd= periderm, c = cortex, cb = cambium, / 2 = secondary tissues, en = endo- 
dermis, pc = pericycle, t r = primary tissues of central cylinder. 
Fig. 5 Transverse section showing secondary thickening entirely outside the 
endodermis. In the secondary tissues, hi four concentric bundles are shown. 
px , one of the protoxylem-groups of the primary cylinder. Other lettering as 
before, x 117. 
Fig. 6. Transverse section showing commencement of secondary growth in 
a transitional region. It is chiefly pericyclic, but a few divisions have taken place 
in the cortex, en , fragments of endodermis. Secondary bundles, derived from the 
pericyclic cambium, are in process of formation. ph v , one of the peripheral 
phloem-groups of the primary cylinder, x 1 1 7. 
Fig. 7. Transverse section passing through part of the insertion of a branch-root. 
At the base of this is a large mass of secondary tissue, formed from pericyclic 
cambium. The endodermis is ruptured. To the right there has been no peri- 
cyclic thickening, but secondary growth has started in the cortex ; b 2 , developing 
bundle. Other lettering as before. The limit of the primary cylinder of the parent 
root is obvious, if traced from pc on the right, x 75. 
Fig. 8. D. Draco , L. Transverse section showing an early stage of cortical 
thickening. Three or four tangential series of cortical cells are taking part in the 
cambial divisions. r = raphide-sac, marking inner limit of primary cortex at this 
