70 



TEXTBOOK OF BOTANY 



J 



STOMATE 



UPPER EPIDERMIS 



PALISADE 1 



MESOPHYLL 

 SPONGY 

 LOWER EPIDERMIS 



VEIN 



Fig. 33. Cross sections of leaves: A-I, sections of embryonic leaves from the 

 terminal bud of a tobacco plant in the order of their ages. The oldest leaf ( I ) in the 

 series was less than one centimeter in length. J, cross section of a small portion of a 

 blade of a mature leaf of tobacco. After G. S. Avery, American Journal of Botany. 



winter some of the leaves in the bud have attained their characteristic 

 pattern in miniature. The cells of the prospective future veins are slightly 

 differentiated, whereas the remaining ones are still similar. Thus far cell 

 division has been the dominating process of development. All cells re- 

 main in complete contact and there are no intercellular spaces. 



Leaf development. With the warmer weather of the following spring, 

 growth is renewed. The stem tip and young leaves expand and press the 

 bud scales apart. During the rapid increase in size, cell enlargement and 

 cell differentiation dominate the growth process, leading to the forma- 

 tion of the distinctly different tissues as seen in microscopic sections. 

 Cell division stops first in the epidermis, next in the spongy mesophyll, 

 and last in the palisade mesophyll. Cell enlargement, however, con- 

 tinues longest in the epidermis; as a result, the spongy cells are pulled 

 into an open meshwork while the palisade cells are but slightly sepa- 

 rated. Meanwhile the vascular bundles of the veins have increased in 

 diameter. Expansion of the leaves follows rapidly after the opening of 

 the buds; they often double their size within twelve hours. In some 

 plants with small leaves the whole development from leaf primordium 



