lo THE STRUCTURE OF ECONOMIC PLANTS 



which may vary in chemical constitution in different walls. Farr 

 and Eckerson (17) state that the ellipsoid bodies in Gossypium are 

 jacketed by a pectic cement, and Farr and Sisson (18) suggest that 

 these visible units satisfactorily account for the X-ray patterns 

 that have been ascribed to the existence of micellar units. This 

 view is not supported by Bailey and Kerr (5), who object to the 

 size of the ellipsoid bodies on the ground that they would exceed 

 the width of more than four lamellae in the cotton fiber. Anderson 

 and Kerr Ql) raise additional objections relative to the cementing 

 substances of pectic material which are described as surrounding 

 the ellipsoidal particles, the migration of these particles into the 

 wall through the selectivity of the permeable membrane, and the 

 orientation of the particles in the wall. 



Chemistry of the Cell Wall. — The chemical composition of 

 the cell wall varies with the age of the cell and with the type into 

 which it ultimately differentiates. In the initial stages of develop- 

 ment, the middle lamella is made up chiefly of pectose with small 

 amounts of cellulose. As the cell attains its full size the pectose 

 rapidly forms an insoluble pectate, commonly calcium pectate, 

 thus making the wall more firm and resistant. The continued 

 deposition of pectose and increasing amounts of cellulose, as well 

 as some hemicelluloses, results in the formation of the primary 

 wall, while the secondary wall is at first composed almost entirely 

 of cellulose and some hemicelluloses, but no appreciable amount of 

 pectic compounds. In parenchymatous cells, this condition may 

 obtain throughout the life of the cell; but, in other tissues, the 

 wall may be further modified by lignification, cutinization, or 

 suberization. 



In lignification, the lignin, which is an amorphous substance, 

 first appears in the middle lamella; and, although it has been sug- 

 gested that the pectic substances of this zone are converted into 

 lignin, the investigations of Kerr and Bailey (15) and of Harlow 

 (xx) indicate that they are still present even in heavily lignified 

 middle lamellae. The primary wall may be lignified and also the 

 secondary one, but it seems probable that there is more than one 

 type of lignin involved. The addition of lignin may increase the 

 mechanical strength of the cell wall without affecting its permea- 

 bility to any great extent; but, when the wall is infiltrated with 

 cutin or suberin, it becomes relatively impermeable to liquids or 

 gases. 



