Tissues in Certain Monocotyledons. 25 
special importance to the direct examination of the stages of 
development as seen in longitudinal sections, in which alone 
the nascent tracheae can be observed in relation to the other 
elements of the bundle and of the ground-tissue. We found 
that the use of the microtome was of essential service. We 
obtained the best results by making continuous series of 
tangential sections through the region of secondary growth. 
In this way we were enabled to examine all the elements in 
each developing bundle, and to compare the various bundles 
at all stages of differentiation. The instrument chiefly used 
was the Cambridge rocking-microtome. The thickness of the 
sections in some series was in. (-005 mm.), in others -oVs 
in. (-0075 mm.). The usual paraffin-method was employed, 
and various stains were used, saflranin being on the whole 
the most successful. The result was of course checked by 
the constant comparison of transverse and radial sections, and 
also by means of maceration. Our work extended to several 
species both of Yucca and Dracaena , but our best results were 
obtained in an unnamed species of Yucca , in which we had 
abundant material of an old stem, reaching at least 3 in. in 
diameter. This stem had been in very active growth when 
preserved, and possessed a wide zone of thickening, with 
secondary bundles in all stages. 
In this species the secondary bundles have the same struc- 
ture as in the Yucca described by Roseler 1 ; they are collateral, 
with the small phloem-group on the outer side, lying in a 
depression of the much larger xylem-mass. The latter con- 
sists mainly of tracheides, with lignified parenchymatous cells 
scattered among them, and lying between the tracheides and 
the phloem. The whole bundle is surrounded by a sheath of 
flattened, partially lignified cells, easily distinguishable from 
the comparatively thin-walled ground-tissue. We found that 
the average number of elements in the transverse section of 
a secondary bundle, taking the mean of twelve countings, is — 
tracheides 36, xylem- parenchyma 13, phloem-elements 17. 
The elements of the sheath average about fifteen in number. 
1 1. c. p. 297. 
