VASCULAR TISSUE. 



439 



being developed in the middle of a spiral fibre. The annular vessel is 

 the first observed in the youngest parts of plants, and when found alone 

 indicates a low degree of organisation ; as shown by its occurrence in 

 Sphagnum, Equisetum, and Lycopodium, which plants, in the ascending 

 scale of vegetation, are almost the first that possess vascular tissue. 



It will be found that spiral fibre occurring with rings marks a 

 higher step in the scale of organising power ; the true spiral more so; 

 and the reticulated and dotted mark the highest ; this being the order 

 in which these several vessels are placed in herbaceous exogens pro- 

 ceeding from within outwards, the differences of structure of the several 

 vessels being indices of the vital energy of the plant at the several 

 periods of its development. In those vessels in which the annular or 

 spiral character of the fibre is departed from, some curious modifi- 

 cations of the above process are to be observed, as in the reticulated 

 vessels met with in the common balsam (Balsamina hortensis). The 

 primary formation of fibre in these vessels is marked by the tendency 

 of the granules to take a spiral 

 course, when it happens that some 

 one of the granules becomes en- 

 larged by the deposition of new 

 matter around it. This becomes a 

 point originating another fibre or 

 branch, which becomes developed 

 by the successive attraction of gra- 

 nules into bead-like strings, taking 

 a contrary direction to the original 

 fibre, forming a cross-bar, or rami- 

 fying, thereby causing the appear- 

 ance by which the vessel is recog- 

 nised. 



In the exogenic vessel, the de- 

 velopment of fibre proceeds in the 

 same manner as in the last example ; 

 but the vessels will be seen to be 

 dotted with a central mark, usually 

 of a red colour, which, when viewed 

 under high power, may be thought 

 to resemble a minute garnet set in 

 the centre of each dot. This red colour is owing to the dot being some- 

 what hollowed or cupped, and the centre only thin membrane. These 

 vessels are best seen in the young shoots of the Willow. In the endo- 



fig. 212. - 



A portion of the epidermis of the sugar- 

 cane, showing the two kinds of cells of 

 which it is composed, magnified 200 

 diameters. 



