204 THE MOVEMENTS OF FLUIDS IN STEMS. 



be met with. Between these two parts, the xylem and phloem, 

 there exists in Dicotyledons and Conifers a series of cells capable 

 of dividing, and known to botanists as the cambium layer ; the 

 special function of the cambium being to form periodically new 

 wood on the inner side and new bast on its outer side. By the 

 activity of the cambium the originally separate fibro-vascular 

 bundles are united into a ring, interrupted only by the medullary 

 rays, primary and secondary, the special plates of cells which keep 

 up a connection between the inner part of the ground tissue (now 

 called the pith), and the outer part of the ground tissue which 

 forms the primary cortex. This condition can be seen in a 

 section of the stem of a young seedling plant above the cotyledons, 

 or in the youngest shoot of a tree. After the first ring of wood 

 and bast has been completed by the cambium, a rest occurs 

 during the winter, and it is only in early summer that the cam- 

 bium again becomes active, and begins to form new wood and 

 bast. It will be evident that this mode of growth causes the 

 separation between the simultaneously formed wood and bast 

 portion to become annually greater and greater, so that in a thick 

 stem the primary wood part is next the pith, while the primary 

 bast portion is on the outside side of the stem, immediately below 

 the corky layer of the bark. The epidermis lasts only for one 

 year, and it is completely replaced in the second season (as may 

 be seen on a second year shoot) by a layer of cork, known to 

 botanists as the periderm. In the willows the corky layer arises 

 from the epidermis itself, but in most plants it is produced by the 

 cells of the ground tissue, either immediately, or at a short dis- 

 tance below the epidermis. After the formation of the periderm 

 the cells (if any) external to it die and become thrown off. New 

 cork cells are formed from a deeper seated layer of cells known as 

 the cork cambium. In some plants repeated layers of cork are 

 formed deeper and deeper in the tissues, giving rise to the com- 

 plex covering known to botanists as the " bark " or rhytidome. 

 In many plants, by the formation of the rhytidome, not only the 

 whole ground tissue forming the primary cortex, but part of the 

 bast portion of the fibro-vascular bundles, may be thrown off. 

 The rhytidome, as above defined, differs from the bark in its 

 ordinary popular sense, as the word bark is commonly used to 

 mean all the part lying externally to the cambium layer, and 

 therefore includes the phloem of the fibro-vascular bundles in 

 addition to the ground tissue with the layers of cork. 



