134 Elementary Biology. 



(p. 197), in that respect assisting the leaves, which are small 

 and inconspicuous. 



The microscopic structure of the stem also shows a like- 

 ness to that of the fern. Externally the stem is covered by 

 epidermis, the cells composing which are elongated and 

 contain chlorophyll grains. Beneath the epidermis are 

 several layers of closely packed cortical cells whose walls 

 are much thickened. Inside this stratum and gradually 

 merging into it lies the general fundamental parenchyma, 

 the cells of which are larger and thinner walled. Plunged 

 in the fundamental tissue are the fibro-vascular strands. 

 These differ from the strands in -the rhizome of the fern in 

 that they are surrounded by a large annular air-space, in 

 the centre of which the fibro-vascular strand is slung by 

 means of anchoring strands composed of small parenchy- 

 matous cells. The fibro-vascular strand itself is very similar 

 in structure to that of the fern, viz. a broad band of scalari- 

 form vessels with a few spiral vessels at either end of the 

 band to represent the xylem, and an enclosing layer of 

 phloem consisting of small parenchymatous cells, the whole 

 enclosed by one or more layers of phloem sheath. 



Leaf. The leaves in Selaginella are of two kinds, small 

 and large. These leaves are arranged one series on one 

 side of the stem, the other bcrics on the other. The ventral 

 leaves, those lying next the ground, are the larger. The 

 leaves of both types are sessile and very simple in structure. 

 They are heart-shape, the broad base being next the stem. 

 In microscopic structure they are covered by an epidermis, 

 the cells of which contain chlorophyll granules, with stomata 

 on the under surface only. The cells composing the 

 epidermis are very similar to the general fundamental 

 tissue of the leaf, which is composed of loosely arranged 

 parenchyma with large and irregular intercellular spaces. 

 Each leaf has one fibro-vascular bundle in the form of a 

 midrib, consisting of the same elements as those constitut- 

 ing the fibro-vascular strands of the stem, only not so plentiful 



