GROWTH SUBSTANCES FOR NORMAL GROWTH 105 



review briefly certain studies on the composition and the micro- 

 scopic structure of the cell wall. 



According to the investigations by Thimann and Bonner 

 (1933), the dry matter of the cell walls of the coleoptile contains 

 about 42 per cent cellulose, the remainder being made up of 

 hemicelluloses, pentosans, and pectins. Recent observations 

 have indicated that the cell wall consists of two different ele- 

 ments, namely, micellae, i.e., little rods, which are probably 

 crystalline, and an intermicellar substance, which fills the spaces 

 between the micellae (Anderson, 1935). According to Heyn 

 (19336; see also Kolkmeijer and Heyn, 1934), dehydration shrinks 

 the cell wall to about one-third of its diameter when wet. Soding 

 (1934) has assumed that the intermicellar substance is of greater 

 volume than the micellae and that it is made up of a \'iscous, 

 colloidal substance. Heyn (1933a, h, 19345) has studied the cell 

 walls of the coleoptile under the microscope and by means of the 

 Rontgen spectrograph. He states that the cell walls are smooth 

 in intact cells, although the inner layers become wrinkled when 

 dehydrated or released from tension ; the outermost layers of the 

 external wall of the epidermis would be shorter than the inner 

 layers when dried out. By means of the Rontgen spectrograph a 

 difference can be found between younger and older cell walls. 

 Heyn concluded the following from these experiments : 



If one assumes that the cellulose macromolecules of the young cell 

 wall have not yet taken on the crystalline form described above, then 

 the important role of the water held in these walls becomes under- 

 standable. In older cells (fibers), aging of the walls is accompanied 

 by progressive dehydration, while more macromolecules are continually 

 taking on this crystalline form, until finally a pure cellulose pattern is 

 obtained. When young walls, consisting of macromolecules, are dried, 

 much more bound water remains between the single molecules not in 

 crystalline form. 



Cell-wall Extensibility. — When considering the physical char- 

 acteristics of the cell wall, its extensibility is of interest. Exten- 

 sion can be either reversible {elastic extensibility) or not reversible 

 (plastic extensibility). According to Pringsheim (1932), one can- 

 not make a sharp distinction between these two types; the 

 amount of wall substance is not changed in either case. Measure- 

 ment of the degree of extensibility in plant organs is a difficult 

 problem; plant cells can change their length easily without 



