Elementary botany . f cells of dissimi-lar form are derived from young growing tissue known as meristem. Meri-stem tissue consists of cells nearly alike in form, with thin cell walls andrich in protoplasm. It is situated at the growing regions of the the higher plants these re-gions in general are three innumber, the stem and rootapex, and the cambium cyl-inder beneath the produced from thestem and root apex are calledprimary, those from the cam-bium secondary. In mostcases the main bulk of theplant is secondary tissue,while in the corn plant it is allprimary. 709. Ori
Elementary botany . f cells of dissimi-lar form are derived from young growing tissue known as meristem. Meri-stem tissue consists of cells nearly alike in form, with thin cell walls andrich in protoplasm. It is situated at the growing regions of the the higher plants these re-gions in general are three innumber, the stem and rootapex, and the cambium cyl-inder beneath the produced from thestem and root apex are calledprimary, those from the cam-bium secondary. In mostcases the main bulk of theplant is secondary tissue,while in the corn plant it is allprimary. 709. Origin of stem tissues.—Just back of themeristem in section of a growing point it can be seen that the cells are undergoing achange in form, and here are organized three formative regions. Theouter layer of cells is called dermatogen (skin producer), because later itbecomes 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. apical dermatogen; p,\. , (After De Bar v.) longitudinal P P Fig. through growing point of stem, d, plerome; periblem between. 360 RELATION TO ENVIRONMENT. (fig. 417). Surrounding the plerome and filling the space between it andthe dermatogen is the third formative tissue called the periblem, which laterforms the cortex (bark or rind), and consists of parenchyma, collenchyma,sclerenchyma, or cork, etc., as the case may be. It should be understoodthat all these different forms and kinds of cells have been derived frommeristem by gradual change. In the mature stems, therefore, there arethree distinct regions, the central cylinder or stele, the cortex, and theepidermis. 710. Central cylinder or stele.—As the central cylinder is organized fromthe plerome it becomes differentiated into the vascular bundles, the pith,the pith rays (medullary rays) which radiate from the pith in the centerbetween the bundles out to the cortex, and the pericycle, a layer of c
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