DIV. I 



MORPHOLOGY 47 



and layers of variously shaped formative cells, which at first retain 

 the general characters of meristematic cells (Figs. 100, 102, 157). 

 Intercellular spaces, absent in the meristem itself, now arise. At a 

 somewhat greater distance from the growing point the characters of 

 the various permanent tissues make their appearance and become 

 more marked basipetally until the mature structure is attained. In 

 this process of tissue-differentiation groups, strands, or layers of cells 

 may retain the meristematic characters and serve as places of origin 

 later for a renewed formation of meristematic and mature tissues. 

 Their power of division may persist throughout the life of the plant. 



In many Monocotyledons the basal region of the internodes remains 

 for a long time meristematic, and serves, in addition to the growing 

 point, as a place of production of permanent tissue. In this way the 

 intercalary growth of these and other plants is brought about. 



2. Secondary Meristems are derived either from the above- 

 mentioned inactive remains of the primary meristem or are newly 

 formed from cells of the permanent tissue, which alter their function 

 and by new cell divisions are transformed into meristematic cells. 

 Their elements resemble those of the primary meristems, but as a rule 

 have the form of elongated or flattened prisms. Such secondary 

 meristems, which get the name CAMBIUM, give origin to cork and to 

 the secondary growth in thickness of woody plants. They form a 

 thin layer of prismatic meristematic cells (Figs. 169, 185) parallel to 

 the surface of the organ at the outside of the cylinder of wood. In 

 the cambium a middle layer of initial cells undergoes continued 

 tangential divisions which cut off daughter cells to both the inside 

 and outside in the radial direction. These cells after some further 

 divisions are transformed into cells of the permanent tissues. 



The new cell walls arising in the cell divisions of a meristem are flat and as a 

 rule, though not without exception, placed at right angles to the pre-existing 

 older walls. Walls more or less parallel to the surface of the organ are termed 

 PERICLINAL, and those at right angles to this ANTICLINAL. 



B. The Permanent Tissues 



The cells of the permanent tissues differ from the meristematic 

 cells in being as a rule larger, with relatively little protoplasm and 

 large vacuoles, and sometimes completely dead. Cell divisions are 

 not usually taking place in them, and the cell walls are variously 

 thickened and often chemically altered. The permanent tissue is 

 composed of a variety of kinds of cells and tissues with diverse 

 functions. It is usually provided with intercellular spaces. 



In developing from the meristem the cells of the permanent tissue 

 enlarge, separate at places from one another, undergo thickening and 

 chemical alterations of their walls, modify or lose their cell contents, 

 and sometimes fuse by dissolution of the partition walls. In enlarging 



