VASCULAK BUNDLE ENDS. 201 



mately becomes so small as even to possess only one annular 

 vascular trachei'de. These thin veinlets lie in the spongy paren- 

 chyma, they anastomose with one another into a fine network, 

 and ultimately end blind within the meshes. We can determine 

 that, almost to the last, the vascular tracheide is accompanied by 

 sac-like elements of the bast ; only very short lateral "twigs " con- 

 sist of vascular tracheides alone. Up to the last the bundle also 

 remains clothed by a single layer of mesophyll cells, elongated in 

 the direction of the vein, and devoid of intercellular spaces. This 

 parenchyma sheath also closes over the end of the vascular bundle, 

 quite gapless. It belongs to the mesophyll, and corresponds with the 

 innermost cortical layer in the stem. Up to the last, the tissue 

 of the central cylinder of the stem, which is represented in the 

 leaf by the vascular bundle and the sclerenchyma sheath which 

 usually accompanies it, and the tissue of the primary cortex, 

 which constitutes the mesophyll of the leaf, are separated from 

 one another. 



Leaf -fall. The fall of foliage leaves in autumn results from 

 the interposition of a separating layer, or what we may call an 

 absciss layer, which is formed earlier or later during the period 

 of vegetation, and which cuts across the articulation of the leaf- 

 stalk. A periderm is also usually formed, by which the scar is 

 closed. We will examine the processes a little more closely in 

 jffisculu-s Hippocastanum (the Horse-Chestnut), during the fall of 

 the leaf ; and as alcohol material serves just as well as fresh, we 

 can be independent of the time of the year. The absciss layer, 

 as well as the cork layer, lie in the position which is clearly 

 visible externally as the boundary between the brown tissue of 

 the cortex and the green tissue of the leaf -stalk ; upwards this 

 boundary strikes the angle which the leaf -stalk forms with the 

 bud in its axis. We cut off the leaf-stalk, with the surrounding 

 parts of the cortex, from the twig, and halve it in a median line. 

 We take now a number of delicate longitudinal sections with the 

 razor, taking care that some of them also cut through a vascular 

 bundle. In such longitudinal sections, prepared from fresh 

 material, and examined in water, the cork layer is at once 

 observable, even with a low power, as a clear brownish streak, 

 between the deeper brown cells of the cortex, and those of the 

 leaf -stalk. In alcohol material the cell- walls of the cortex and of 

 the leaf-stalk are colourless. The cork layer is clearly reddish- 

 hrown, especially on the cortical side. It consists of six or eight 



