b58 



A TEXT-BOOK OF BOTANY. 



are associated with the protective celluloses. They dissolve or 

 swell in water, are colored blue (as in flaxseed) or yellowish with 

 iodine, and are stained with alcoholic or glycerin solutions of 

 methylene blue. 



Mineral cellulose walls are composed of cellulose and vari- 

 ous inorganic substances, as silica, calcium oxalate, or calcium 

 carbonate. These are more commonly found in the cell-wall of 

 the lower plants, as Algae, Fungi, and Equisetacese. Calcium car- 

 bonate and silica also occur in the cystoliths of the various genera 

 of the Moraceae and Acanthaceae (Fig. 113). 



From what has just been said of the chemical composition and 

 structure of the cell-wall, it is seen that it consists of lamellae or 



Fig. 129. I, cross-section of a bast fiber of Begonia as seen by means of the micro- 

 polariscope, and showing stratification of the wall. 2, polariscopic view of a spheio-crystal 

 of inulin in Helianthus tuberosus. — After Dippel. 



layers of different substances, and in no case does it consist of 

 but a single substance; but for convenience we speak of a wall as 

 consisting of cellulose, lignin, or suberin, meaning thereby that 

 the wall gives characteristic reactions for these substances. 



Phytomelane, an intercellular, carbon-like substance. It is 

 a black, structureless substance, found only in the Compositae, 

 being distributed around the sclerenchymatous fibers and stone 

 cells in a number of fruits. It has also been found in the lignified 

 cells of roots and stems (Fig. 131), and occasionally is found in 

 the parenchyma cells of Inula. In the latter it occurs more or less 

 crystalline, sometimes in the form of short needles or rods (Fig. 

 132). According to Hanausek (" Untersuchungen iiber die 

 Kohlf ihnliche Afasse der Kompositen "), phytomelane occurs in 

 a largv number of genera in the Compositae. It arises in the middle 

 lamellae and has a high content in carbon, ranging from 69.76 per 



