176 INTRODUCTION TO CYTOLOGY 



Trichosanthes anguina that the position of the spiral and annular thicken- 

 ings is determined by peripheral bands of cytoplasm separated by more 

 vacuolate regions (Barkley, 1927). 



Composition of the Cell Wall. — The principal constituents of the 

 completed cell walls of vascular plants are polysaccharides: cellulose, 

 hemicelluloses, and pectins. With these are often associated other 

 substances, notably lignin, suberin, and cutin.^^ The distribution of 

 these components in the various layers of the wall has been the subject 

 of many investigations.^" The earlier workers, who relied chiefly on 

 staining reactions, concluded that the middle lamella consists of pectic 

 compounds, while the secondary and tertiary layers are composed chiefly 

 of cellulose with or without lignin, pectin, and other additional materials. 

 Subsequent studies with chemical methods have shown that the middle 

 lamella, although it consists chiefly of pectin in one or more of its forms in 

 parenchyma, is principally lignin in woody tissues, and that staining 

 reactions cannot be safely relied upon as criteria of chemical composition. 

 What becomes of the original pectins in the young wall during its lignifi- 

 cation is still somewhat uncertain. Harlow finds that the secondary 

 layers of softwoods are appreciably lignified, while those of hardwoods, 

 with few exceptions, have practically no lignin. Tertiary layers are 

 often lignified. 



The secondary and tertiary layers are made up of numerous thin 

 lamellae which appear to differ in the relative amounts of cellulose and 

 pectin present (van Wisselingh) as well as in physical properties. In the 

 wall of the flax fiber the layers have been shown to consist of spirally 

 arranged fibrils, the fibrils in contiguous layers running in opposite 

 directions (D. B. Anderson, 1927). In cotton the direction of coiling 

 changes at intervals along the fiber (Balls, 1923). 



Suberin and cutin are varying mixtures of certain organic acids 

 present in part in the form of fats (Priestley, 1921). They are distin- 

 guished more by location than by composition, suberin appearing in 

 the walls of periderm cells (cork), while cutin occurs as a cuticle on the 

 epidermis and often in patches in the walls of subepidermal cells. In 

 the resin canals of conifer leaves the suberin is said to be deposited on the 

 primary layer and rarely on the secondary one (Gauba, 1926). Lee and 

 Priestley (1924) have ascribed the formation of plant cuticle to certain 



^^ For an extensive account of walls and their constituents, see van Wisselingh 

 (1924). See also Czapek (1913), Molisch (1913), von Wettstein (1921o), Gleisberg 

 (1921), Grafe (1911, 1922), and Gortner (1929). Among the earlier papers on this 

 subject are those of Payen (1842), Fremy (1859), Kabsch (18636), Wiesner (1864, 

 1886), Mangin (1888-1893), Schulze (1890-1894), Gilson (1890), van Wissehngh 

 (1888, 1892, 1895), and Allen (1901). 



1^ Among recent works, see especially Tupper-Carey and Priestley (1923), Ritter 

 (1925, 1928), Harlow (1927, 1928a6c, 1931), D. B. Anderson (1926, 1927, 1928), and 

 Scarth, Gibbs, and Spier (1929). 



