TISSUES IN HIGHER PLANTS. 53 



case the order is always the same. Exogens are dicotyledonous 

 plants and are characterized by having their stems sharply 

 divided into bark and central wood cylinder, while endogens, 

 or monocotyledonous plants, have no sharply -defined bark and 

 wood core. 



To get an idea of the various kinds of tissues and the par- 

 ticular order of arrangement always met with in exogens, the 

 stem of the common Geranium serves very well. The stem of 

 most any other exogen would answer, as the Bittersweet, Elder, 

 Willow, Sycamore, Maple, Yellow-Parilla, etc. 



Make several thin cross-sections of a common Geranium stem, 

 free-hand or by the microtome ; place one on a slide, add a few 

 drops of chlor-zinc-iodine solution and cover. Examine with 

 low power. 



The first thing that strikes one is that the cells differ in size 

 and shape, in compactness of arrangement, in thickness of cell- 

 walls, in contents, besides that some stain blue while others 

 stain brown. 



Going from the exterior towards the center the very first 

 layer of cells is the 



1. Epidermis or external bounding tissue, a single tier of 

 closely laid and similar cells, interspersed with hairs and having 

 their outer walls thickened into a cuticle. If the stem be too 

 old, the epidermis may not be present. 



2. Beneath the epidermis are several tiers of tabular, brick- 

 shaped cells in radial rows, the cells of the outer layers are 

 empty and their walls stain brown, while the inner cells may 

 contain protoplasm and the walls show some blue color. This 

 is the cork layer. 



3. Collenchyma, next to the cork, consisting of cells quite 

 different in shape and arrangement from the cork cells, with 

 cellulose walls, as shown by the blue color with chlor-zinc- 

 iodine, rounded or polygonal and thickened at the angles where 

 the cells join. The cells are rich in protoplasm. They are not 

 as well developed nor as conspicuous in the Geranium as in 

 some other plants. 



4. The next layer of cells is ordinary parenchyma or ground 

 tissue,, a broad zone of large cells, with walls very thin and 

 uniform and stained blue (cellulose), rich in protoplasm and 

 starch contents, globular in shape, with small angular inter- 

 spaces at the angles where the cells meet. 



5. Bast fibres, next to the parenchyma, a zone of much 

 smaller, angular, very thick-walled cells, stained a deep brown 

 (lignified walls), compactly arranged and free from proto- 

 plasmic and starchy contents. The bast fibres are dead and act 

 only as mechanical tissue. The cell-walls are stained brown 

 like those of the cork cells, which might indicate that they are 

 composed of the same material. That this is not so may be 

 shown by adding to a fresh section a drop of phloroglucin 



