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 



