4. ORIGIN OF THE TISSUES. =708. Meristem tissue.=—The various tissues consisting of cells of dissimilar form are derived from young growing tissue known as _meristem_. Meristem tissue consists of cells nearly alike in form, with thin cell walls and rich in protoplasm. It is situated at the growing regions of the plants. In the higher plants these regions in general are three in number, the stem and root apex, and the cambium cylinder beneath the cortex. Tissues produced from the stem and root apex are called _primary_, those from the cambium _secondary_. In most cases the main bulk of the plant is secondary tissue, while in the corn plant it is all primary. [Illustration: Fig. 417. Section through growing point of stem, _d_, dermatogen; _p_, plerome; periblem between. (After De Bary.)] =709. Origin of stem tissues.=—Just back of the apical meristem in a longitudinal section of a growing point it can be seen that the cells are undergoing a change in form, and here are organized three formative regions. The outer layer of cells is called _dermatogen_ (skin producer), because later it becomes the epidermis. The central group of elongating cells is the _plerome_ (to fill). This later develops the _central cylinder_, or _stele_, as it is called (fig. 417). Surrounding the plerome and filling the space between it and the dermatogen is the third formative tissue called the _periblem_, which later forms the cortex (bark or rind), and consists of parenchyma, collenchyma, sclerenchyma, or cork, etc., as the case may be. It should be understood that all these different forms and kinds of cells have been derived from meristem by gradual change. In the mature stems, therefore, there are three distinct regions, the central cylinder or stele, the cortex, and the epidermis. [Illustration: Fig. 418. Concentric bundle from stem of Polypodium vulgare. Xylem in the center, surrounded by phloem, and this by the endodermis. (From the author’s Biology of Ferns.)]