96 PRACTICAL BOTANY 



cuticle (brown). Here and there may be found stomata, with 

 two small guard-cells, and two subsidiary cells. 



b. The sclerenchyma consists of cells with thick, highly 

 refractive walls, which stain yellowish brown with chlor-zinc- 

 iodine (lignified). 



c. The parenchyma consists of cells with thin cellulose walls, 

 At the angles where the cell-walls meet are intercellular spaces. 

 The external layers have abundant protoplasm with chloro- 

 phyll-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 w ^h 

 the angle towards the centre of the stem : of these the two 

 smaller are developed first. Compare sections of young stems. 



The vessel nearest the centre of the stem has usually annular 

 thickening : in old stems it is partially surrounded by an inter- 

 cellular space, while the rings often become detached, in which 

 case the vessel is not easily seen in transverse sections. Next 

 to this is a vessel which has commonly a spiral thickening : the 

 remaining two have thinner walls with pitted marking, and 

 large cavity. 



Surrounding the pitted vessels, and between them, are 



ii. A number of tracheides with pitted lignified walls, and no 

 cell-contents. Surrounding the intercellular space above de- 

 scribed is 



iii. A group of parenchymatous cells with thin cellulose 

 walls. These may be regarded as xylem parenchyma. 



The phloem portion of the bundle lies between the limbs of 

 the V-shaped xylem, and is easily recognized by the thin 

 cellulose walls characteristic of soft bast. It consists of 



iv. Elements with large cavities, in which transverse septa 

 (sieve-plates) often occur : these elements are the sieve-tubes. 



v. Smaller cells (cambiform) between the sieve-tubes. 



Surrounding the above tissues of the xylem and phloem is a 

 sheath of sclerenchyma. On its internal side may be found 



