59 
Tissues in Certain Monocotyledons. 
(1) Aristea corymbosa, Benth., in common no doubt with the 
few other shrubby species of Irideae, forms an indefinite 
amount of secondary tissue by means of cambium, which 
continues active during the whole life of the plant. 
(2) The tissues formed centrifugally, on the inner side of the 
cambium, consist of secondary concentric bundles, imbedded 
in ground-tissue ; on the outer side of the cambium a large 
amount of secondary cortex is formed. The latter is wholly 
parenchymatous. 
(3) The xylem of the secondary bundles consists chiefly of 
tracheides, each of which arises, as in Yucca and Dracaena , by 
the enormous elongation of a single cell. 
(4) The cambium arises in the pericycle, and is a new forma- 
tion ; the cambial divisions do not begin until some time after 
the development of the primary vascular cylinder is completed. 
(5) The inner zone of secondary tissues is characterized by 
its very crowded bundles. The cambium which forms this 
zone has no definite initial layer ; each cambial cell undergoes 
a few centrifugal tangential divisions, then its activity ceases, 
and the divisions are taken up by an adjacent cell to the 
exterior. Consequently the elements of the inner zone do 
not show a regular radial arrangement. 
(6) After a time (possibly, under normal circumstances in 
the second year) the divisions become more regular, a cam- 
bium with a definite initial layer is established, and the forma- 
tion of the outer zone of thickening begins, and continues 
without limit. This zone is characterized by its scattered 
bundles imbedded in comparatively thin-walled ground-tissue. 
After this zone has begun to develope the formation of secon- 
dary cortex commences. 
(7) Successive layers of periderm are formed, by which the 
whole of the primary cortex is eventually removed, the sub- 
sequent periderms arising in the outer part of the secondary 
cortex. 
The occurrence of secondary thickening in this little group 
of Irideae, a group which is so narrowly limited both syste- 
matically and geographically, appears to us to be a fact of 
