HORMONAL REGULATION OF PLANT CELL GROWTH 189 



rate on turgor pressure. That the inhibition is nearly linear with 

 mannitol eoncentration lias tended to suggest that growth rate is 

 proportional to, or at least increases with, increasing turgor pressure, 

 as a passive stretching miglit be expected to, although as previously 

 indicated the actual turgor pressures are not known with any cer- 

 tainty. 



An interesting feature of the cell wall which seems to be con- 

 nected with the stresses and strains it is subject to, is the orientation 

 of its structural elements. The cell wall contains what looks like a 

 "meshwork" of submicroscopic "microfibrils" of a-cellulose ( in higher 

 plants largeh- /S-l,4-D-glucosan), which appear to be embedded in 

 an amorphous "matrix" of non-cellulosic polysaccharides. Because 

 of the hio;h tensile streno;th of a-cellulose, and other considerations, 

 the tendency has been to ascribe to the microfibrils a determining 

 role in the mechanical properties of the cell wall; sometimes they 

 are likened to the reinforcing rods of concrete. 



The lateral walls of typical more or less cvlindrical cells which 

 are elongating in the direction of the cylinder axis show, as a general 

 rule (with exceptions), microfibrils oriented predominantly in the 

 transverse direction with respect to the axis of growth. It is easy 

 to see how this reinforcing could result in the observed longitudinal 

 enlargement of the cell, even though the forces imposed by turgor 

 tend to stretch it in all directions. Electron microscopists have also 

 noticed that elongating cell walls commonly have less well oriented 

 or even longitudinally oriented ( parallel with axis of growth ) micro- 

 fibrils at the outer surface. This has been explained (see Roelofsen, 

 1959) by assuming that during growth the cell adds microfibrils in 

 close to transverse orientation at the inner surface, next to the pro- 

 toplast, while the alreadv existing cell wall simply stretches, pas- 

 sively bringing about a gradual reorientation of the older, outer 

 microfibrils to the longitudinal direction as the cell wall becomes 

 stretched more and more in that direction. 



This brings up the old and persistent question, as to whether 

 svnthesis of the cell wall, which during growth goes on usually at 

 a rate comparable with the rate of extension of the cell, occurs at 

 the inner surface of the cell wall ("apposition") or within and 

 throughout the thickness of the cell wall ("intussusception"). Much 

 of the literature is pervaded by the belief that the growing (pri- 

 mary) wall is synthesized bv intussusception. An evident possibility 

 for the mechanism of cell growth is that internal synthesis of the cell 



