162 PKACTICAL BOTANY 



than at the centre ; also that the position of the parts 

 of the bundles relatively to the centre of the section 

 is usually uniform. 



Treat a section with chlor-zinc-iodine : put on a 

 high power, and examine in detail the several tissues 

 above named. 



a. The epidermis appears as a definite layer of 

 cells of unequal size, without intercellular spaces. 

 Note a well-marked cuticle (brown). Here and 

 there may be found stomata, with two small guard- 

 cells, and two subsidiary cells : the structure and 

 development of the stomata will be studied in the 

 leaf; p. 175. 



&. The sclerenchyma consists of cells with thick, 

 highly refractive walls, which stain yellowish brown 

 with chlor-zinc-iodine (lignified). Note that it does 

 not occur immediately below the stomata, but, as 

 usual, there is there an intercellular space (air 

 chamber). 



c. The parenchyma consists of cells with thin 

 cellulose walls (blue with chlor-zinc-iodine). At the 

 angles where the cell-walls meet are intercellular 

 spaces. The external layers have abundant protoplasm 

 with chlorophyll-corpuscles. These are less frequent 

 in the inner layers, while in the central parenchyma 

 the protoplasm is hardly appreciable. 



d. For the minute study of the vascular bundles 

 select one of the largest central bundles. The section 

 must be thin. The most prominent elements in the 

 bundle are 



i. Four large vessels of the xylem, arranged like 

 a V, with the angle towards the centre of the stem : 



