180 



XIII. THE STEMS OF PTERIDOPHYTES. 



elements. The vascular bundle is bounded externally by a 

 starch-containing layer of cells (am) which is to be recognised 

 as a pericycle. Within and laterally the bundle is clad with 

 ground-tissue cells, devoid of intercellular spaces, though not- 

 otherwise specially marked. To the pericycle succeeds the 

 endodermis (e) which therefore, like the pericycle, surrounds the 

 entire ring of bundles. The endodermis is unilamellar, and as a 



rule shows clearly 

 the dark shadings 

 on the radial walls; 

 sulphuric acid 

 shows these up 

 sharply as cutin- 

 ised bands, while 

 the rest of the 

 tissue generally 

 (except the rings, 

 etc., of the xylem) 

 are left colourless 

 and gradually dis- 

 solved. Outside 

 the endodermis is 

 a large- celled cor- 

 tex, having large 

 vallecular air- 

 passages, on radii 



FIG. 7'2. Part of a cross-section through the internode of alternating with 



a barren shoot of M. arvense, showing a vascular bundle and , v i 11 mr_ 



the surrounding tissue, a, isolated rings from vascular es - 



tracheides ; a', crushed annular trachei'des, representing the surface of the stem 

 protoxylem ; r, annular tracheides at the sides of the bast ; 



d, carinal air-passage ; vg, vascular bundle sheath ; m, pith ; shows projecting 



am, starch sheath (pericycle) ; e, endodermis ; c, cortex ; vl, r :;i a p Q . tViPQPv^rrpc 

 portion of a vallecular passage (x 90) 



lie opposite the 



bundles, the epidermis being here devoid of stomata and pro- 

 tected by strings of sclerenchyma fibres. Under these strings 

 lies the chlorophyll-containing tissue, loose parenchyma, which 

 extends on both sides of the string to the epidermis. Here, on 

 the slopes of the ridges, are found the stomata, while the middle 

 portion of the furrow has again an epidermis devoid of stomata, 

 and with a sclerenchymatous hypoderma, though more feebly 

 developed. 



The physiological relations of these tissues are easy to deter- 



