40 PLANT GROWTH SUBSTANCES 



properties. This can be illustrated from studies made on the secondary 

 wall of the cotton hair (3). Cotton fibers of commerce are twisted, ribbon- 

 hke structures, whereas the same fibers before drying for the first time 

 are hollow cylinders. During the initial dehydration twists or convolu- 

 tions first appear and once formed are irreversible. Before the initial 

 drying the cellulose does not give an X-ray diffraction pattern or at the 

 most, only a faint indication of one. This must mean that crystallization 

 or hydrogen bonding of cellulose does not occur at the time of deposition 

 but takes place chiefly during the initial dehydration. The cellulose of 

 the primary walls of cotton hairs likewise gives an X-ray diffraction 

 pattern only after drying. Heyn (7) has reported a similar situation 

 in the parenchyma of Avena coleoptiles. Undried cotton fibers show 

 considerable plasticity but when stretched under water, crystallization 

 of the cellulose occurs even without drying. The artificial stretching of 

 primary cells reported by Bonner (4) unquestionably involves not only 

 a reorientation of the cellulose but also crystallization. Dried primary 

 walls do not show the plastic behavior of undried membranes. It is 

 difficult to see how a primary wall composed of a network of microfibrils 

 undergoing a rapid increase in surface area as a result of a sharp increase 

 in turgor, could avoid crystallization of the cellulose. Nevertheless, it 

 is known that crystallization does not take place. Therefore, it is still 

 highly improbable that undried cellulose could form a skeletal network 

 with the elastic extensibility of the primary membrane. 



In order to have a better understanding of the primary wall, it is 

 necessary to consider the structure and properties of pectic substances. 

 Our knowledge of the manner in which these substances occur in the 

 wall is still somewhat vague. Preston (11) has recently considered that 

 they are encrusting substances in the same category as lignin. Frey- 

 Wyssling (5) does not mention the pectic substances in his recent dis- 

 cussion of the growth of the primary membrane. Materials in the primary 

 wall grouped under the term "pectic substances" are unquestionably 

 mixtures, including arabans, galactans and possibly other hemicelluloses. 

 However, true pectic substances are apparently restricted to the primary 

 walls and intercellular substance of higher plants and do not seem to be 

 present elsewhere. Pectic substances are now recognized to be composed 

 of straight, long chained molecules of anhydrogalacturonic acid units 

 (8). The carboxyl groups of the polygalacturonic acid may be partly 

 esterified by methyl groups, in which case the substance is called "pectin," 



