[Illustration: Fig. 5. Young Stem, Magnified 18-1/2 Diameters, Showing Primary and Secondary Bundles. _By Courtesy of Mrs. Katharine Golden Bitting._ E, epidermis, the single outside layer of cells. C, cortex, the region outside of the bundles. HB, hard bast, the black, irregular ring protecting the soft bast. SB, soft bast, the light, crescent-shaped parts. Ca, cambium, the line between the soft bast and the wood. W, wood, segments showing pores. MR, medullary rays, lines between the bundles connecting the pith and the cortex. MS, medullary sheath, the dark, irregular ring just inside the bundles. P, pith, the central mass of cells.] In young plant cells, the whole cavity of the chamber is filled with protoplasm, but as the cells grow older and larger, the protoplasm develops into different parts, one part forming the cell wall and in many cases leaving cavities within the cell, which become filled with sap. The substance of the cell wall is called cellulose (cotton and flax fibers consist of almost pure cellulose). At first it has no definite structure, but as growth goes on, it may become thickened in layers, or gummy, or hardened into lignin (wood), according to the function to be performed. Where there are a group of similar cells performing the same functions, the group is called a tissue or, if large enough, a tissue system. When cells are changed into new forms, or "differentiated," as it is called, they become permanent tissues. These permanent tissues of the tree trunk constitute the various parts which we have noticed, viz., the rind, the pith and the wood. The essentially living part of the tree, it should be remembered, is the protoplasm: where there is protoplasm, there is life and growth. In the stems of the conifers and broad-leaved trees--sometimes together called exogens--this protoplasm is to be found in the buds and in the cambium sheath, and these are the growing parts of the tree. If we followed up the sheath of cambium which envelopes a stem, into a terminal bud, we should find that it passed without break into the protoplasm of the bud. In the cross-section of a young shoot, we might see around the central pith or medulla, a ring of wedge-shaped patches. These are really bundles of cells running longitudinally from the rudiments of leaves thru the stem to the roots. They are made of protoplasm and are called the "procambium strands," Fig. 6. [Illustration: Fig. 6. Three Stages in the Development of an Exogenous Stem. P, pith; PB, primary bast; SB, secondary bast; C, cambium; PR, pith ray; PW, primary wood; SW, secondary wood; PS, procambium strands. _After Boulger._]