280 PRACTICAL BOTANY 



We can now obtain a clear idea of the vascular 

 system of a mature shoot of Lycopodium clavatum 

 there is in the first place a central vascular cylinder, 

 which traverses the shoot longitudinally, and from the 

 periphery of it single bundles of small size are given 

 off, which take an obliquely ascending course in radial 

 planes, and each of them enters a lea By further 

 comparison of longitudinal and transverse sections, it 

 may be ascertained that the masses of xylem in the 

 central cylinder have the form of flattened plates, the 

 planes of which are approximately horizontal in the 

 living plant. They are sometimes separate from one 

 another, sometimes joined towards the centre of the 

 stem, and it is on the margins of these plates that the 

 bundles of the leaf-trace are inserted. 



Examine the radial longitudinal sections under a 

 high power, and observe 



i. That the cells of the epidermis, and the scleren- 

 chymatous portion of the cortex, are elongated and 

 prosenchymatous, while those of the thin-walled 

 band of the cortex are shorter, and tend to a paren- 

 chymatous form. The walls of the cortical cells are 

 pitted. 



ii. Of the vascular tissues the xylem is the most 

 prominent ; its chief constituents are of prosenchyma- 

 tous form, with lignified walls : the latter show the 

 scalariform marking which is due to the regular 

 arrangement of elongated pits with their longer axes 

 placed horizontally : each of these pits shows a double 

 contour, and transitional forms will be found from the 

 elongated to circular pits, the latter presenting an 

 appearance very similar to that of the bordered pits 



