THE MECHANISM OF THE ACTION 125 



cent, protein 12 per cent (Thimann and Bonner, 1933). 

 Since the dry weight is only one third of the fresh weight, 

 cellulose constitutes only 14 per cent of the fresh cell walls. 

 Nevertheless, it is structurally probably the most important 

 constituent, and even the thin growing parenchyma walls 

 have a well-developed continuous cellulose skeleton (Bonner, 

 1935). On this account, experimental study of the cell wall 

 has been principally directed at the cellulose. The tech- 

 niques used are based upon its crystalline structure, and 

 are therefore those of x-ray photography and the polarizing 

 microscope. The cellulose consists, at least partly, of sub- 

 microscopic crystalline aggregates or micelles, whose orienta- 

 tion determines the stretching properties of the wall. In 

 the epidermis they are mostly arranged parallel to the long 

 axis, an arrangement which Frey-Wyssling (1935) has 

 called ''fiber-like structure" (Figure 41b) (Heyn, 1933a, 

 1934a; Bonner, 1935). The micelles themselves, being 

 crystalline, do not stretch, and correspondingly it is found 

 that the epidermis stretches very little when suspended in 

 auxin solution (Bonner, 1934). In the growing parenchyma, 

 however, the micelles are oriented more nearly perpendicular 

 to the long axis — Frey-Wyssling's "tube-structure" — (Fig- 

 ure 41 d). Further, since they occupy only a small part of 

 the volume of the cell wall but yet form a continuous skele- 

 ton, these micelles, which are probably smaller than those 

 in the epidermis, must be attached to one another in some 

 way. We thus have a network, the openings being rhom- 

 boidal, with the long axis of the rhomboid perpendicular 

 to the long axis of the plant. Under tension these rhomboids 

 will change shape, the long axis finally coming parallel 

 to the direction of tension. However, in growing coleoptiles 

 the orientation remains on the whole perpendicular to the 

 long axis of the plant, as shown by the optical measurements 

 of Bonner (1935), and therefore growth cannot be a simple 

 stretching. Growth must therefore be accompanied by a 

 sliding of the points of attachment and the laying down of 

 new micelles between the old in the same direction (see 



