STRUCTURE OF LEAF 



295 



mesh or leaf-gap corresponds in position to the attachment 

 of a leaf. The latter is supplied by the numerous fine strands 

 passing out from the margin of the mesh. These leaf-traces can 

 be recognised, in a transverse section of the rhizome (Fig. 

 162, B), as arcs of small strands (/./.) occupying the lobes that 

 represent the leaf -bases (l.b.) In the case of the Bracken (Pteris) 

 a series of transverse sections, cut in the region of the node, 

 shows that the leaf-base is supplied by several strands passing 

 off from the outer ring. The gap thereby created is filled by 

 steles from the inner series, which pass out, through a break 

 in the sclerenchyma-ring, on the side towards the leaf-trace. 

 The petiole in both these Ferns is thus traversed by a large 

 number of strands, normally forming an arch, but in certain 

 genera (e.g. Gleichenia] the leaf-stalk, like the stem, contains but 

 a single stele. 



In its general structure the leaf conforms to the ordinary 

 dorsiventral type. Since many Ferns grow in shady situations, 

 the epidermis often contains chloroplasts, and the palisade tissue 

 is not uncommonly poorly developed or the mesophyll even 

 spongy throughout (cf. Fig. 165). The ultimate branches of the 

 concentric strands traversing the petioles become collateral 

 within the pinnae, owing to development of the phloem only on 

 the lower side. 



The roots of most Ferns are diarch, and owe their frequent 

 wiry character to the development of an exceedingly thick- 

 walled sclerenchymatous cortex, but otherwise do not differ 

 appreciably from those of higher plants. The tetrahedral apical 

 cell, by whose divisions the tissues of stem and root are formed, 

 has already been described on p. 18. 



In the ordinary course of events the Fern-plant sooner or 

 later commences to produce spores. These are developed within 

 small, stalked, usually brown-coloured sporangia, almost in- 

 variably borne on the under-surfaces of the fronds, which in 

 some few cases are completely covered by them (e.g. Rusty- 

 Back). Most commonly, however, they are arranged in numerous 

 separate groups, or son, each usually comprising a considerable 

 number of sporangia arising from a slight central swelling of 

 the leaf-tissue, known as the placenta (Fig. 165, pi.). 



Such sori are well seen in the Common Polypody (Polypodium 



