1022 



A TEXTBOOK OF THEORETICAL BOTANY 



the palisade tissue exposes from 1-6-3 -5 times as much free cell surface as 

 the spongy tissue. This is due to the form of the cells which lends itself to 

 close ranking. In spite of variations in the size of the palisade cells, the 



Fig. 1 010. — Sahola kali. Transverse section of 

 leaf showing centric structure. 



palisade ratio, which is the ratio of the number of palisade cells to epidermal 

 cells in a unit area, is remarkably constant and is used as a diagnostic character 



A B C 



Fig. ioii. — Diagrams showing the structure of unifacial leaves. A, Biitottius umbellatus. 

 B, Iris wartani. C, Acidanthera platypetala. {After Velenovsky.) 



in pharmacognosy (Fig. 1012). This constancy is due to the mode of growth 

 in the young leaf. Cell division ceases first in the epidermis, but the epidermal 

 cells continue to expand, as marginal growth of the leaf continues, after the 

 mesophyll cells have ceased to grow. The latter are, therefore, drawn apart 

 and the intercellular spaces are thus created. Obviously, therefore, wider 

 spacing of the palisade cells will be correlated with larger epidermal cells, 

 and the constancy of the palisade ratio is analogous to the constancy of the 

 stomatal index. 



