392 



PART IV. CLASSIFICATION. 



out intercellular spaces. In correlation with this each stele is suspended, 

 by delicate trabecular cells developed from the endodermis, in an air- 

 chamber': each vascular bundle going to a leaf is in a similar chamber 

 which communicates in the lamina with the external air through the 

 stomata. Each stele is surrounded, towards the air-chamber by a peri- 

 cycle consisting of one or sometimes two layers of cells. The stele is 

 circular or oval in transverse section, it usually consists of two or more 

 bundles arranged concentrically, the bast forming a peripheral layer 

 and the wood-bundles joining in the centre. 



Rhizophore and root are both monostelic, and without air-shambers : 

 the stele contains but one bast- and one wood-bundle. 



B 



FIG. 234. Etnbryogeny of Sela.ginella Martensii (after Pfeffer). Two isolated embryos at 

 different stages, A Younger embryo ( x 610) B older ( x 165) : s suspensor ; c 1 c 2 cotyledons ; 

 ststem; I young foliage-leaves ; hyp hypocotyl ; r root; /so-called foot. 



The bundles are all closed ; there is no secondary growth in thickness. 



The leaves are very simple in structure ; they have a midrib with a 

 single vascular bundle : the epidermal cells contain chloroplastids which, 

 like those in the other cells, are large and are present in small numbers 

 (sometimes only one) in the cells. The stomata are usually confined to 

 the under surface, on the sides of the midrib. 



The growth in length of the stem is effected by an apical growing-point 

 which has, in some species (e.g. S. Martensii), a two-sided or three-sided 

 apical cell ; whilst in others (e.y. S. Lyalli, Pervillei, etc.) it consists of 



