90 



Growth 



in length is chiefly apical and seems to be governed by a group of cells 

 at the tip, just under the epidermis-essentially a meristem. This produces 

 the central tissues of the young primordial axis, or midrib, and growth of 

 the epidermis keeps step with it. Such apical growth soon ceases, how- 

 ever, and later growth is diffuse. 



The development of the tobacco leaf studied by Avery ( 1933 ) may be 

 taken as a typical example of the growth of a determinate organ (Fig. 

 4-24 ) . After the axis is about 1 mm. long and while still it is very narrow, 

 growth of the lamina begins on both sides of this axis, pushing out like a 

 wave of developing tissue. It increases faster in the middle than at either 



d2^> 



A, 



^b=^4=^s 



Fig. 4-24. Early developmental stages of a tobacco leaf, from a young primordium 

 (upper left) to later ones where lamina and veins are being formed. (From G. S. 

 Avery. ) 



end, and this produces the characteristic leaf shape. The rate of growth, 

 as a result both of the division of the cells and of their increase in size, 

 is greater in certain dimensions than in others. Avery contrasts the growth 

 differences resulting from such polarized growth, primarily due to differ- 

 ences in plane of cell division, with localized differences in rate of divi- 

 sion. Differences in cell shape due to differential cell expansion have little 

 share in over-all shape changes in the organ as a whole. In most leaves, 

 growth in the various dimensions of the blade, whatever its cellular basis, 

 is unequal, so that blade form changes somewhat during development. 

 These changes are under morphogenetic control, however, and show close 



