FILICALES 



485 



in protecting the curled tip of the leaf, since in Ferns apical growth 

 is long continued, and the apical tissues arc delicate. The stem and 

 leaves, especially while young, are often densely covered either with 

 hairs (Osmunda) or chaffy scales (Dryopteris), which protect the 

 young parts against drought, but are liable to fall away later. 



In their general construction Ferns resemble Flowering Plants. 

 They have a superficial epidermis, and a conducting system of vascular 

 tissue, embedded in ground-tissue which is parenchymatous : but often 



Fig. 375- 

 Transverse section of a fossil-Fern, Botryopteris cylindrica, showing a protostele 

 with solid central core of xylem, and peripheral phloem. This is a fossil from 

 the Palaeozoic Period, and it illustrates how perfect the preservation of structure 

 may sometimes be in fossils of very early times. 



it also encloses strands or islands of hard brown sderenchvma, while 

 hard stony or horny sheaths frequently form the surface of stem and 

 leaf-stalk. The epidermis and ground tissue call for no detailed 

 description. The chief interest lies in the vascular system. In 

 ancient fossil Ferns, such as Botryopteris, in many primitive living 

 Ferns, and generally in young sporelings there is a simple stele of a 

 type called a " protostele," having a solid xylem-cure. and phloem 

 surrounding it. This is believed to have been the primitive strueture 

 for them all. It is well shown in Botryopteris (Fig. 375). Occasionally 

 this state may be retained through life [Hymenophyllum, Lygodium). 



