100 VEGETABLE HISTOLOGY. 



bundles of the veins with the xylem always towards the upper 

 side and the phloem towards the lower side of the leaf. The 

 upper and lower faces of a leaf can always be told by noting the 

 position of the xylem and phloem of the bundles of the leaf. 



CENTRIC LEAF. This type of leaf is symmetrical, i. e., the 

 structure on one side is the same as on any other side. Palisade 

 cells are not present. Centric leaves are terete, acicular or 

 succulent, and, occasionally, flattened leaves belong to the type. 



Most any pine needle will illustrate the type; also leaves of 

 Lady's Slipper, Sweet Flag, Hyacinth, Daffodil. 



Make cross-sections of a pine needle by holding it between 

 pith and cutting through the latter. If necessary, clear them in 

 carbolic acid, chloral hydrate or Labarraque's solution. 



The leaf is flat on one side, which is the upper or ventral, and 

 nearly semi-circular on the other. The epidermis is a single 

 layer of thick-walled cells which possesses stoinata on all sides 

 of the leaf. 



Next to the epidermis are two or three layers of thickened 

 fibrous cells, and next to these comes the parenchyma of the leaf, 

 consisting of thin-walled cells, whose walls have been infolded, 

 forming a variety of parenchyma known as folded. The cells 

 contain chlorophyll bodies. Arranged at nearly equal intervals 

 in the parenchyma are about five secretion reservoirs, in which 

 the circle of secreting cells is enclosed by one of thick-walled 

 cells. 



A bundle sheath separates the central portion from the rest 

 of the section, next to which are parenchyma cells surrounding 

 two collateral bundles in the center of the section. 



Apply phloroglucin and hydrochloric acid and note result; 

 also iodine solution. 



Fig. 62. Cross-section of a pine needle (centric leaf), a, bast fibres of bundle; b, 

 stoma ; c. epidermis ; d, secretion channel ; e, air space ; f, folded paren- 

 chyma ; g, bundle sheath ; h, parenchyma ; i, phloem, or soft bast of bundle ; 

 k, xylem of bundle (reduced, from Bastin). 



