3 68 



A TEXT-BOOK OF BOTANY. 



The petiole generally shows the structure of the midrib as far 

 as concerns the mestome-strands, but possesses furthermore a 

 more or less strongly developed parenchyma, the cells of which 



FIG. 213. Development of stomata on leaves of Sedum purpurascens: in A very 

 early stages of growth, and B nearly completed stoma. In B are shown a stoma with two 

 guard cells, three neighboring cells and two of the epidermal cells (e); the numbers in B 

 correspond to those in A and show the origin of the several cells from the division of a single 

 epidermal cell. After Sachs. 



are colorless, thin-walled and which may often be traced to the 

 leaf-blade itself, where it surrounds the stronger veins, causing 



FIG. 214. Transverse section through a stomata showing how by a slight difference 

 in the tension the pore is either opened or closed; the dark lines show contour of cells 

 when -open, the light lines show when they are closed. After Schwendener (See Haberlandt. 

 Physiologische Pflanzenanatomie). 



them to project as ribs and to be much thicker in cross-section 

 than the adjoining chlorenchyma. 



From a histological point of view the leaf structure of 



