50 ELEMENTS OF PLANT ANATOMY. 
almost if not quite impossible, in any single instance, to trace 
back the periblem layers to a distinct cell or set of cells, which 
retain their position at the axis and act as initial cells. The 
same is true regarding the plerome tissue. It is not inconceiv- 
able nor impossible that the point of vegetation is differentiated 
in this way, but the majority of facts now known point rather 
to the opposite opinion. 
Therefore at the present time the weight of evidence appears 
to be against the Hanstein theory of the point of vegetation, 
and it is generally believed that in all phanerogamic stems there 
is asmall cluster of cells irregularly arranged at the apex, which 
have always retained their meristematic nature, that from this 
group are derived all the subsequent cells and that just below 
it the cells differentiate into the meristems from which the three 
systems are developed. ‘These cells at the apex have been named 
by the Germans Urmeristem or original meristem. Whatever 
view may be held regarding their arrangement the use of terms 
is not thereby affected if it is only held in mind that the tissues 
spoken of as periblem and plerome are somewhat indefinite as 
to their origin and limitation. 
Having now described the origin and partial development 
of the principal tissues and systems, we may next begin the 
study of the component elements of these systems in their 
finished or mature condition. 
3. Hpidermal System. 
This includes those cells which have the function of taking 
in food materials, of throwing off or excreting waste products, 
and finally of protecting the inner tissues from injurious out- 
side influences. In reference to the last function it is divided 
into primary epidermis, or that derived from the dermatogen 
and continuing only for a short time, and secondary epidermis, 
or periderm. As the latter occurs only in connection with 
