ANATOMY OF TISSUES. 107 
there is a distinct dermatogen there is no apparent separation 
of periblem and plerome ; and in other instances where such 
separation is plainly indicated by direction of walls and arrange- 
ment of cells, it is quite impossible to trace back the meristems 
to distinct initial groups. Although much time and labor have 
been spent on the study of the apical regions of phanerogams 
in order to ascertain their exact manner of development, no 
definite, satisfactory results have as yet been obtained. It is 
now believed by many that the urmeristem of such stems, from 
the Conifers upward, consists of a group of cells with no definite 
order of arrangement and no regular method of division. The 
terms introduced by Hanstein are those in common use when 
referring to these regions, and there is no danger of their 
misapplication if these facts are held in mind. With this 
explanation we may now pass on to the consideration of the 
different kinds of stems. 
MONOCOTYLEDONOUS STEM. 
A cross-section through such a stem discloses at once the 
peculiar structure common to this class; that is, the bundles 
appear scattered singly and without order over nearly the whole 
cross-section. They are usually more numerous toward the 
periphery, and in many instances there is left a central pith 
quite free from bundles. This either remains during the life 
of the plant, or becomes destroyed so as to leave a hollow stem. 
The bundles are always closed and generally collateral, with a 
tendency to the concentric arrangement, as both phloem and 
xylem are frequently surrounded by a bast cylinder. The 
primary epidermis is well developed, and in certain cases is 
replaced by periderm. The ground system is mostly parenchy- 
matic and sometimes appears to be divided into rind and pith 
cells, as there is often a small zone near the circumference com- 
paratively free from bundles. Although there are several 
yarieties of stems of this class in reference to the arrangement 
