16 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



which form the outer covering of the plumule of germinating cereal 

 grains. After the coleoptiles have reached a length of 1 cm,, further 

 growth is exclusively by vacuolation, i.e. increase in water content 

 per cell, leading to an increase in vacuole size and increase in 

 cell length. Under carefully controlled conditions, a whole batch of 

 coleoptiles can be persuaded to grow at the same rate. This provides very 



-3S 



cm. 



■30 



Wall per 

 10,000 



■2-5 



20 



IS 



Length (cm.)- 



FiG. 3(6). The relation between wall weight (gm.) per unit coleoptile length and 

 coleoptile length (Preston and Clarke). Conventions as in Fig. 3(o). After a length of 

 1 cm. has been reached, the wall weight decreases continually apart from a halt lasting 

 from a length of 3 cm. to a length of almost 5 cm. in crops grown in the dark at 10°C. 

 Note again that the upward turn at the end of the light 23°C. series is associated 



with a cessation of growth in length. 



convenient material for a study of cell elongation, since samples can 

 be harvested at intervals for analysis. The results of one such analysis 

 are presented in Fig. 3. It will be clear that the wall weight per coleop- 

 tile, and therefore per cell, increases regularly throughout the whole 

 growing phase (Fig. 3(a)) so that there can be no doubt but that new wall 

 material is laid down during growth. There is, on the other hand, the 

 further peculiar fact that the wall weight per unit length of coleoptile, 

 and therefore approximately per unit cell length, decreases continually 

 over the growth phase (Fig. (3Z>)). Such wall deposition as occurs during 

 growth, therefore, fails to keep pace with dimension changes. This is a 

 point which must be considered later on. At the moment it is merely 

 to be noted that, in the particular case of oat coleoptiles, while the 



