124 PRACTICAL BOTANY 



5. Internally are radiating rows of vessels of xylem, separated 

 by rays of tissue, which, together with the central pith, show a 

 collenchymatous thickening of the cellulose walls. 



Cut successive transverse sections at the upper limit of the 

 pulvinus, and on into the normal petiole, and note how the cen- 

 tral vascular mass separates into distinct bundles, forming an 

 irregular ring round the central, thin- walled or even fistular pith. 

 The structure of the pulvinus is thus well adapted to its function 

 as a hinge, or organ of movement. 



B. LAMINA. 



I. Take a piece of the lamina of the leaf of the 

 Sunflower, including the apex : it is important that it 

 should be previously bleached by treatment with alcohol : 

 warm it gently in a mixture of dilute glycerine and 

 potash, and mount in glycerine : examine with a low 

 power, and observe 



1. The midrib, with its strongly marked vascular 

 bundle, running up to the apex of the leaf, where it 

 terminates abruptly in a mass of glandular parenchy- 

 matous tissue. 



2. Lateral branch-bundles the ribs or nerves 

 passing off from it, and forming a network by frequent 

 anastomoses, while some of them run up into and 

 terminate in the serrate projections of the margin of 

 the lamina in a manner similar to the midrib as above 

 described. 



3. Smaller branch-bundles, sometimes showing blind 

 endings in the parenchyma which fills the meshes of 

 the network. 



Vascular skeletons of the simple, coriaceous leaf of Buxus 

 sempervirens may be prepared, so as to show the entire network, 



