ANATOMY OF TISSUES. 49 
where this method of development occurs. Sanio held an en- 
tirely different opinion relative to this question. He believed 
that the cambium ring exists first and from this ring the vascular 
bundles are formed. Later investigations tend to substantiate 
the views of both authors, and it is not at all improbable that 
the two methods occur in plants of different kinds.! 
The continued development of this ring constitutes what is 
known as secondary growth, a subject which will be taken up 
later on. During the processes just described, a gradual tran- 
sition from the primary meristems to the three definite systems 
of tissues has been completed. The word system is used to 
denote sets of tissues which are continuous for long distances. 
These are called epidermal, vascular, and ground systems, and 
are supposed to be derived from the three primary meristems, 
dermatogen, periblem, and plerome. As before intimated it 
has been found difficult to determine the exact limit between 
periblem and plerome, and it is therefore impossible in such 
cases to trace back the origin of the vascular and ground systems 
to one or the other of these two meristems. In other cases where 
this limit between periblem and plerome has been considered 
evident, it has been found that the cambium cells which give 
rise to the vascular system originate in part from the periblem 
and in part from the plerome, while the remaining meristematic 
cells of both classes give rise to the ground, or fundamental 
tissue. 
The principal facts known in reference to the epidermal 
system are such as to offer no decided objection to the theory 
above explained ; at the same time they are not such as to require 
the assumption of a special initial cell or cells for the renewal 
of the dermatogen.” With reference to the two other systems, 
however, the case is quite different; here it has been found 
1 Naegeli: Ueber das Wachsthum, etc. Beitriige zur wiss. Botanik, 1858. 
Sanio: Vergleichende Untersuchungen. Bot. Zeit. Numbers 11-15; 47-61, 
1863. 
