44 ELEMENTS OF PLANT ANATOMY. 
centres of growth, others occur which give rise to the lateral 
organs of the stem. In studying the subsequent changes of the 
plant, in order to learn the origin and development of the differ- 
ent tissues and systems, it will be best to select a single centre 
of growth, punctum vegetationis, and follow its history alone, 
leaving the other parts of the plant for special consideration. 
It has been found extremely difficult to ascertain definitely 
the exact method of growth and cell-division at these centres; 
and consequently there is great uncertainty and difference of 
opinion regarding the meristematic tissues of the growing tips 
of higher plants. In fact, it is now believed by most authorities 
that the forms described in our text-books as illustrating the 
manner of growth of these plants are exceptional cases and do 
not represent the general or more frequent method of develop- 
ment. However this may be, the terms used in describing 
these plants have become so universal in the literature pertain- 
ing to plant anatomy, and so incorporated with it, that they 
must be explained and thoroughly understood from the old 
standpoint, before we are able to take up this study in the litera- 
ture of more recent date. So far the above description answers 
equally well for all young embryonic plants belonging to the 
phanerogams. From this point onward the development of the 
stem apex of a dicotyledonous embryo affords the best illustra- 
tions of the tissues to be described. 
At that time in the life of the young plant when localization 
of growth first occurs, the cells are all similar in form and size, 
and without definite arrangement. The first appearance of 
variation in shape of cells, and of any order of arrangement 
occurs very soon after this at the stem apex. Here the new 
cells derived from the lttle cluster of meristematic cells at or 
about the apex are said to separate into three groups which 
were named by Hanstein Dermatogen, Periblem, and Plerome. 
These three groups of cells are also meristematic in nature and 
they give rise directly to the lasting tissues of the stem. They 
