176 XIII. THE STEMS OF PTEKIDOPHYTES. 



ning more or less parallel to, the steles, which thus form an inner 

 and an outer series. The outer steles are supported on their 

 outer side, immediately adjoining the endodermis, by similar 

 sclerenchyma fibres, which evidently have a mechanical function. 

 In the interior of the leaf-stalk the relations are similar, excepting 

 that here there is also a hypodermal ring of reddish-brown 

 sclerenchyma fibres, which underlies the epidermis, and forms a 

 true cortical sclerenchyma, 



The longitudinal section through the rhizome, or the leaf- 

 stalk, shows most prominently the scalariform vessels. The end 

 walls of these are sharply inclined, with ladder-like bordered pits, 

 partially broken through. It is now easy to determine that on 

 the side walls which separate two vessels the horizontally elon- 

 gated pits are bordered on both sides, and that the closing 

 membrane possesses a thickened torus, but on the wall of those 

 vessels which adjoin a wood-parenchyma cell the border is only 

 unilaterally developed (on the side towards the vessel), and that 

 the closing membrane has no torus. The longitudinal section 

 may also have cut through one or other of the spiral protoxylem 

 elements, and the sieve-plates of the sieve-tubes may also, with 

 very careful examination, be disclosed, being made more recog 

 nisable by numerous refractive granules. We can make the 

 sieve-plates somewhat clearer with the aid of aniline blue, and 

 determine that the terminal sieve-plates are very oblique, and divided 

 by thickened bands into numerous sieve-areas. Besides these, the 

 lateral walls of the tube also bear roundish sieve-pits. Near the 

 sieve-tubes can be recognised the bast-parenchyma with abundant 

 protoplasmic contents and nucleus ; in contact with the vessels, 

 the starch-containing, comparatively short cells of the wood- 

 parenchyma ; while of much the same shape as these are the 

 starch-containing cells of the peripheral starch sheath. The red- 

 brow^, long, pointed sclerenchyma fibres of the ground-tissue 

 show delicate pits in their walls. 



Stem of Lycopodium. A somewhat higher degree of com- 

 plexity is shown by the axial stele of the species of Lycopodium 

 (Club-mosses) ; but the structure will not appear so difficult 

 after we have made a careful study of the stele of the Fern, 

 In Lycopodium we] have, in fact, a coalescence of a number of 

 steles, each like that of the Fern, into a gamostele. For study 

 we select Lycopodium complanatum ; but any other species will 

 serve equally well, since in all species of Lycopodium the relations 



