ACEOGENOUS OR ACOTYLEDONOUS STEM. 



73 



upper end. The sheath of the lower internode embraces the base 

 of the internode above it (fig. 139). The vascular bundles unite 

 to form a hollow cylinder iu the stem. In fig. 140 is shown the 



Fig. 139. 



structure of a vascular bundle of Equisetum hyemale, with 

 a hollow cavity or lacuna, I, round which are large annular and 

 spiral vessels, I v, smaller vessels, s v, and peculiar cells, c v ; which, 



Fig. 138. Ly&ypodvum clavaty/m, a species of Club-moss, showing a branch, I, covered with 

 mimite pointed leaves, from which proceeds a stalk bearing at its extremity two spikes, f^ 

 consisting of modified leaves with fructification. Fig. 139. Fructification of a species of 

 Horse-tail (Egmsetitm maaAmvm). The stalk is surrounded by a series of membranous 

 sheaths, s s, which are fringed by numerous sharp processes or teeth. The fructification, 

 /, is at the extremity of the shoot, in the form of a pyramidal mass of polygonal scales, 

 bearing spores on their under surface. The fructification in some species is on the same 

 branch with the leaves, while in others it is on a separate branch. 



