1036 



summary is drawn^ arc penned to show the best treatment of leaves 

 or petals prior to a microscopical examination ; the author then 

 proceeds — 



" 39, The vascular bundles of a leaf are continued into its petiole, 

 and from thence they pass into the stem, where they become dispersed, 

 some are continuous with the spiral vessels situated around the pith, 

 and others with the vessels of the layer of wood last formed. These 

 vessels, in their passage through the bark, are contained in large 

 canals, situated between the liber and the cuticle, in which they appear 

 to lie naked. Some of the vessels in the petiole do not pass directly 

 into the stem, but either become intermixed with a quantity of loosely 

 connected cells, or anastomose with the vessels of the adjacent 

 bundles ; these can be very well seen in the fig-tree, by making a 

 vertical section through the petiole at its insertion into the stem. The 

 canals which transmit the vessels appear to be continuous with spaces 

 of various sizes, continued all along the bark of the stem, and situated 

 at different distances from the surface. Their walls or immediate 

 boundaries consist of rows of cells piled one row upon another; some- 

 times they are smooth and at others they are covered with small 

 whitish granules. Many of the passages seem to become filled up 

 more or less completely with clusters of new cells, which have a pale 

 greyish tint, and resemble wood cells in their form and general charac- 

 ters ; they are also acted upon in a manner similar to such cells by the 

 tincture of iodine. In order to trace distinctly the passage of the 

 vessels of the petiole into those of the wood, thin, vertical and hori- 

 zontal sections must be made through the attachment of the petiole with 

 the stem, and afterwards treated with acetic acid in the manner describ- 

 ed. As the vessels descend from the attachment of the petiole into 

 the wood, they lose somewhat the character of true spirals, and become 

 more like barred ducts. Probably a difference in the mechanical 

 conditions under which vessels are placed in the leaves and in the 

 stem, renders it necessary that the former should be more perfect 

 spirals than the latter. 



" 40. The manner in which vessels are formed from cells can be 

 very well seen by examining plants during their growth, and contrast- 

 ing the same structures at difTerent periods of their development. The 

 petals of the potato, taken a little before the flowers have expanded, 

 and previously rendered transparent by the employment of acetic acid, 

 show well the manner in which vessels increase in length. In the 

 colourless parts of these petals, bundles of long but very narrow cells 

 may be observed marking out the place for the future vessels ; and, at 



