254 



BOTANY 



B 



conspicuous in the root and leaf. These are at first composed of thin-walled 

 elements (procambium), but later some of them begin to show the characters of 

 the elements found in the older vascular bundles these being met with for the 

 first time among the Pteridophytes. The first recognizable elements are short 

 spiral or reticulate tracheids, which appear near the junction of the young bun- 

 dles in the middle of the embryo, and develop from this point toward the apices 

 of the elongating members. 



Vascular Bundles. The completed vascular bundle of the young stem shows 

 a central mass of tracheary tissue, some of whose cells have the scalariform 

 markings found in the trapheids of the older stem. Around these are several 

 rows of cells forming the phloem, but at this stage perfect sieve-tubes cannot be 

 made out. The endodermis, or bundle-sheath, is also much less evident than in 

 the older sporophyte. 



The tracheary tissue of the cotyledon is composed entirely of spiral tracheids, 

 >and, like the stem-bundle, the sieve-tissue and endodermis are poorly developed. 



The bundle of the primary root is 

 "monarch " ; i.e. there is a single strand 

 of primary wood, and as in the other or- 

 gans, the other elements of the bundle 

 are not well developed. 



Ground - Tissue. The tissue lying 

 around the vascular bundles is usually 

 known as the ground-tissue. This 

 remains very much like the original 

 parenchyma, but in the lamina of the 

 leaf it forms the spongy mesophyll, 

 which is the principal green tissue of 

 the plant, and its spaces communicate 

 with the external atmosphere by means 

 of the stomata developed in the epi- 

 dermis. 



THE MATURE SPOKOPHYTE 



The sporophytes of the various 

 Ferns differ much in size. In 

 some of the Hymenophyllaceae 

 there is a slender creeping stem 

 with upright leaves less than a 

 centimetre in length. Some of 

 the Cyatheacese are Tree-ferns 

 with upright stems ten to fifteen 

 metres in height, and leaves four 

 to five metres long. Ferns of 

 temperate regions usually have a 

 subterranean stem, which forms 

 an upright or creeping rhi- 

 zome. This may branch mono- 

 may be developed from the old 

 of this adventitious budding is 



FIG. 218. Adiantum pedatum. A, 

 rhizome, with young leaf, I, and base 

 of an older one, I' ; x, stem-apex, 

 slightly enlarged. B, leaf-segment, 

 showing the venation, and sori, s, 

 covered with the marginal indusium. 



podially, or adventitious buds 

 leaf-bases. A conspicuous case 



